Check for leading zero in js_isarrayindex that caused false positives.

We're supposed to check whether a string turned into an integer and back
is itself, while also returning the value of the integer. We were
unintentionally allowing integers with leading zero through.

jsrun.c

@@ -444,6 +444,11 @@ void js_rot(js_State *J, int n)
 int js_isarrayindex(js_State *J, const char *p, int *idx)
 {
 	int n = 0;
+
+	/* check for '0' and integers with leading zero */
+	if (p[0] == '0')
+		return (p[1] == 0) ? *idx = 0, 1 : 0;
+
 	while (*p) {
 		int c = *p++;
 		if (c >= '0' && c <= '9') {