rtld: check for overflow in parse_integer()

Reviewed by:	dim, emaste, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56151

libexec/rtld-elf/rtld.c

@@ -6492,7 +6492,11 @@ parse_integer(const char *str)
 		if (c < '0' || c > '9')
 			return (-1);
 
+		if (n > INT_MAX / RADIX)
+			return (-1);
 		n *= RADIX;
+		if (n > INT_MAX - (c - '0'))
+			return (-1);
 		n += c - '0';
 	}