Removed ugly size_t less than zero check.

CLA: trivial.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2674)

crypto/mem.c

@@ -162,7 +162,7 @@ void *CRYPTO_malloc(size_t num, const char *file, int line)
     if (malloc_impl != NULL && malloc_impl != CRYPTO_malloc)
         return malloc_impl(num, file, line);
 
-    if (num <= 0)
+    if (num == 0)
         return NULL;
 
     FAILTEST();