Sort the conditional statements

This patch will sort the conditional statements to reduce the conditional branches
by skipping if statements that do not need to be executed depending on the value of `first`.

−       | before   | after    | result
--      | --       | --       | --
Oj.load | 325.594k | 341.356k | 1.048x

### Environment
- Linux
  - Manjaro Linux x86_64
  - Kernel: 6.12.4-1-MANJARO
  - AMD Ryzen 9 8945HS
  - gcc version 14.2.1
  - Ruby 3.4.1

### Code
```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'oj'
end

json = '{"a":"Alpha","b":true,"c":12345,"d":[true,[false,[-123456789,null],3.9676,["Something else.",false],null]],"e":{"zero":null,"one":1,"two":2,"three":[3],"four":[0,1,2,3,4]},"f":null,"h":{"a":{"b":{"c":{"d":{"e":{"f":{"g":null}}}}}}},"i":[[[[[[[null]]]]]]]}'

Benchmark.ips do |x|
  x.time = 10
  x.report('Oj.load compat') { Oj.load(json, mode: :compat) }
end
```

### Before
```
$ ruby json_load.rb
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
Warming up --------------------------------------
      Oj.load compat    32.352k i/100ms
Calculating -------------------------------------
      Oj.load compat    325.594k (± 1.8%) i/s    (3.07 μs/i) -      3.268M in  10.039265s
```

### After
```
$ ruby json_load.rb
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
Warming up --------------------------------------
      Oj.load compat    34.274k i/100ms
Calculating -------------------------------------
      Oj.load compat    341.356k (± 3.6%) i/s    (2.93 μs/i) -      3.427M in  10.056686s
```

ext/oj/parse.c

@@ -681,26 +681,28 @@ void oj_parse2(ParseInfo pi) {
     pi->cur = pi->json;
     err_init(&pi->err);
     while (1) {
-        if (0 < pi->max_depth && pi->max_depth <= pi->stack.tail - pi->stack.head - 1) {
+        if (RB_UNLIKELY(0 < pi->max_depth && pi->max_depth <= pi->stack.tail - pi->stack.head - 1)) {
             VALUE err_clas = oj_get_json_err_class("NestingError");
 
             oj_set_error_at(pi, err_clas, __FILE__, __LINE__, "Too deeply nested.");
             pi->err_class = err_clas;
             return;
         }
         next_non_white(pi);
-        if (!first && '\0' != *pi->cur) {
-            oj_set_error_at(pi,
-                            oj_parse_error_class,
-                            __FILE__,
-                            __LINE__,
-                            "unexpected characters after the JSON document");
-        }
-
-        // If no tokens are consumed (i.e. empty string), throw a parse error
-        // this is the behavior of JSON.parse in both Ruby and JS.
-        if (No == pi->options.empty_string && 1 == first && '\0' == *pi->cur) {
-            oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character");
+        if (first) {
+            // If no tokens are consumed (i.e. empty string), throw a parse error
+            // this is the behavior of JSON.parse in both Ruby and JS.
+            if (RB_UNLIKELY('\0' == *pi->cur && No == pi->options.empty_string)) {
+                oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character");
+            }
+        } else {
+            if (RB_UNLIKELY('\0' != *pi->cur)) {
+                oj_set_error_at(pi,
+                                oj_parse_error_class,
+                                __FILE__,
+                                __LINE__,
+                                "unexpected characters after the JSON document");
+            }
         }
 
         switch (*pi->cur++) {
@@ -761,7 +763,7 @@ void oj_parse2(ParseInfo pi) {
         case '\0': pi->cur--; return;
         default: oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character"); return;
         }
-        if (err_has(&pi->err)) {
+        if (RB_UNLIKELY(err_has(&pi->err))) {
             return;
         }
         if (stack_empty(&pi->stack)) {