Check for signs in unsigned numbers and fail if we see one.

-1 is not an unsigned number.  For that matter, neither is +1;
"unsigned" means "without a sign", and they both have signs.

ANSI C's strto{whatever} routines - even the ones that supposedly are
for "unsigned" values - and the GLib routines modeled after them allow a
leading sign, so we have to check ourselves.

Change-Id: Ia0584bbf83394185cde88eec48efcdfa316f1c92
Reviewed-on: https://code.wireshark.org/review/17511
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wsutil/strtoi.c

@@ -53,6 +53,13 @@ gboolean ws_strtou64(const gchar* str, guint64* cint)
 	gchar* endptr;
 	guint64 val;
 
+	if (str[0] == '-' || str[0] == '+') {
+		/*
+		 * Unsigned numbers don't have a sign.
+		 */
+		errno = EINVAL;
+		return FALSE;
+	}
 	errno = 0;
 	val = g_ascii_strtoull(str, &endptr, 10);
 	if ((val == 0 && endptr == str) || (*endptr != 0)) {