pubkey-speed: Add sanity check for the number of rounds

The allocated buffer for the signatures is based on this, which LGTM
doesn't like.

scripts/pubkey_speed.c

@@ -56,6 +56,10 @@ int main(int argc, char *argv[])
 	}
 
 	rounds = atoi(argv[3]);
+	if (rounds < 0 || rounds > (2^26))
+	{	/* arbitrary limit to the number of chunk_t/sigs that fit into 1 GiB */
+		usage();
+	}
 
 	if (streq(argv[2], "rsa"))
 	{