Fix wrong comment

`ruby_strdup` does not return NULL.
[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62253 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -220,8 +220,7 @@ real_ms_time(void)
     return tv.tv_usec / 1000.0 + tv.tv_sec * 1000.0;
 }
 
-/* Make and return copy of STR in the heap.  Return NULL in case of a
-   failure.  */
+/* Make and return copy of STR in the heap. */
 #define get_string ruby_strdup
 
 static void