Fix a signed/unsigned warning

This causes a compilation failure when using --strict-warnings in 1.0.2
and 1.0.1

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>

crypto/asn1/asn1_lib.c

@@ -175,7 +175,7 @@ static int asn1_get_length(const unsigned char **pp, int *inf, long *rl,
         *inf = 0;
         i = *p & 0x7f;
         if (*(p++) & 0x80) {
-            if (i > sizeof(ret) || max < i)
+            if (i > sizeof(ret) || max < (long)i)
                 return 0;
             while (i-- > 0) {
                 ret <<= 8L;