Avoid expensive mb_valid check.

Encoding name			Returns != 0 when
-----------------------------   --------------------------------------
check_mb_big5			0xA1 <= c <= 0xF9
check_mb_cp932			0x81 <= c <= 0x9F || 0xE0 <= c <= 0xFC
check_mb_eucjpms		c >= 0x80
check_mb_euckr			c >= 0x80
check_mb_gb2312			0xA1 <= c <= 0xF7
check_mb_gbk			0x81 <= c <= 0xFE
check_mb_sjis			0x81 <= c <= 0x9F || 0xE0 <= c <= 0xFC
check_mb_ucs2			always returns length 2
check_mb_ujis			c >= 0x80
check_mb_utf16			complicated
check_mb_utf32			always returns length 4
check_mb_utf8_valid		c >= 0x80
check_mb_utf8mb3_valid		c >= 0x80
my_ismbchar_gb18030		0x81 <= c <= 0xFE

The ASCII-compatible encodings, i.e. cases where the c >= 0x80 check is
sufficient, have the minimum char length == 1.

Author: nielsdos

ext/mysqlnd/mysqlnd_charset.c

@@ -840,8 +840,10 @@ PHPAPI zend_ulong mysqlnd_cset_escape_slashes(const MYSQLND_CHARSET * const cset
 		char esc = '\0';
 		unsigned int len = 0;
 
-		/* check unicode characters */
-		if (cset->char_maxlen > 1 && (len = cset->mb_valid(escapestr, end))) {
+		/* check unicode characters
+		 * Encodings that have a minimum length of 1 are compatible with ASCII.
+		 * So we can skip (for performance reasons) the check to mb_valid for them. */
+		if (cset->char_maxlen > 1 && (*((zend_uchar *) escapestr) > 0x80 || cset->char_minlen > 1) && (len = cset->mb_valid(escapestr, end))) {
 			/* check possible overflow */
 			if ((newstr + len) > newstr_e) {
 				escape_overflow = TRUE;