Fixed possible buffer overflow in php_base64_decode();

# This bug doesn't appear to be harmful for now,
# so I won't merge it into branches...

ext/standard/base64.c

@@ -140,7 +140,7 @@ unsigned char *php_base64_decode(const unsigned char *str, int length, int *ret_
 	}
 
 	/* run through the whole string, converting as we go */
-	while ((ch = *current++) != '\0') {
+	while ((ch = *current++) != '\0' && length-- > 0) {
 		if (ch == base64_pad) break;
 
 	    /* When Base64 gets POSTed, all pluses are interpreted as spaces.