fixed #43998 Two error messages returned for incorrect encoding for mb_strto[upper|lower]

ext/mbstring/php_unicode.c

@@ -276,6 +276,11 @@ MBSTRING_API char *php_unicode_convert_case(int case_mode, char *srcstr, size_t
 	size_t i;
 	enum mbfl_no_encoding _src_encoding = mbfl_name2no_encoding(src_encoding);
 
+	if (_src_encoding == mbfl_no_encoding_invalid) {
+		php_error_docref(NULL TSRMLS_CC, E_WARNING, "Unknown encoding \"%s\"", src_encoding);
+		return NULL;
+	}	
+
 	unicode = php_mb_convert_encoding(srcstr, srclen, "UCS-4BE", src_encoding, &unicode_len TSRMLS_CC);
 	if (unicode == NULL)
 		return NULL;