Do not depend on the evaluation order of C arguments

The evaluation order of C arguments is unspecified. `RSTRING_LEN(value)` would fail if the conversion to a String by `StringValuePtr(value)` is not done yet. Coverity Scan found this issue.

Author: mame

ext/psych/psych_emitter.c

@@ -304,11 +304,12 @@ static VALUE scalar(
  tag = rb_str_export_to_enc(tag, encoding);
  }
 
+ const char *value_ptr = StringValuePtr(value);
  yaml_scalar_event_initialize(
  &event,
  (yaml_char_t *)(NIL_P(anchor) ? NULL : StringValueCStr(anchor)),
  (yaml_char_t *)(NIL_P(tag) ? NULL : StringValueCStr(tag)),
- (yaml_char_t*)StringValuePtr(value),
+ (yaml_char_t*)value_ptr,
  (int)RSTRING_LEN(value),
  plain ? 1 : 0,
  quoted ? 1 : 0,