Don't recompute the strlen in pm_eval_make_iseq

We've already computed the length of the string, so we can reuse it.

vm_eval.c

@@ -1709,7 +1709,7 @@ pm_eval_make_iseq(VALUE src, VALUE fname, int line,
                     continue;
                 }
 
-                pm_string_constant_init(scope_local, name, strlen(name));
+                pm_string_constant_init(scope_local, name, length);
             }
         }