Use safer return values in uECC_verify()

This is a first step in protecting against fault injection attacks: the
attacker can no longer change failure into success by flipping a single bit.
Additional steps are needed to prevent other attacks (instruction skip etc)
and will be the object of future commits.

The return value of uECC_vli_equal() should be protected as well, which will
be done in a future commit as well.

library/pk.c

@@ -591,7 +591,7 @@ static int uecc_eckey_verify_wrap( void *ctx, mbedtls_md_type_t md_alg,
 
     ret = uECC_verify( keypair->public_key, hash,
                        (unsigned) hash_len, signature, uecc_curve );
-    if( ret == 0 )
+    if( ret != UECC_SUCCESS )
         return( MBEDTLS_ERR_PK_HW_ACCEL_FAILED );
 
     return( 0 );