Add some missing bounds checks in signature verification routines.

[originally from svn r9978]

sshdss.c

@@ -72,6 +72,9 @@ static Bignum get160(char **data, int *datalen)
 {
     Bignum b;
 
+    if (*datalen < 20)
+        return NULL;
+
     b = bignum_from_bytes((unsigned char *)*data, 20);
     *data += 20;
     *datalen -= 20;

sshrsa.c

@@ -842,6 +842,8 @@ static int rsa2_verifysig(void *key, char *sig, int siglen,
 	return 0;
     }
     in = getmp(&sig, &siglen);
+    if (!in)
+        return 0;
     out = modpow(in, rsa->exponent, rsa->modulus);
     freebn(in);