isUTF8_CHAR() Check ptr before dereferencing

It is legal to call this function, though not so done in core, with
empty input.  By swapping two conditions in the same 'if', we check if
empty before trying to access it.

Author: khwilliamson

inline.h

@@ -1140,7 +1140,7 @@ Perl_isUTF8_CHAR(const U8 * const s0, const U8 * const e)
 
 #if defined(UV_IS_QUAD) || defined(EBCDIC)
 
-    if (NATIVE_UTF8_TO_I8(*s0) == 0xFF && e - s0 >= UTF8_MAXBYTES) {
+    if (e - s0 >= UTF8_MAXBYTES && NATIVE_UTF8_TO_I8(*s0) == 0xFF) {
        return is_utf8_char_helper(s0, e, 0);
     }