Separate RSTRING_PTR from a function call

Do not apply RSTRING_PTR, a macro which evaluats its argument
multiple times, on a function call.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66377 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/date/date_core.c

@@ -6661,7 +6661,9 @@ tmx_m_of(union DateData *x)
 static char *
 tmx_m_zone(union DateData *x)
 {
-    return RSTRING_PTR(m_zone(x));
+    VALUE zone = m_zone(x);
+    /* TODO: fix potential dangling pointer */
+    return RSTRING_PTR(zone);
 }
 
 static const struct tmx_funcs tmx_funcs = {