lib/hcrypto: mpz2BN return NULL if mp_ubin_size(s) returns zero

If mp_ubin_size(s) returns zero then mp_to_ubin() will fail and
not return MP_OKAY.  If MP_OKAY is not returned, NULL is returned
to the caller of mpz2BN().

This change avoids the unnecessary memory allocation and function
calls.  It also removes a dereference after null warning from
coverity.

Change-Id: I52ff2c166964e41cb4eef1dac637904bf2bf13bf

lib/hcrypto/rsa-ltm.c

@@ -456,8 +456,11 @@ mpz2BN(mp_int *s)
     void *p;
 
     size = mp_ubin_size(s);
+    if (size == 0)
+	return NULL;
+
     p = malloc(size);
-    if (p == NULL && size != 0)
+    if (p == NULL)
 	return NULL;
 
     ret = mp_to_ubin(s, p, SIZE_MAX, NULL);