avutil/hmac: Check av_hmac_alloc() return code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/hmac.c

@@ -245,6 +245,8 @@ int main(void)
     /* SHA-2 */
     while (i <= AV_HMAC_SHA512) {
         hmac = av_hmac_alloc(i);
+        if (!hmac)
+            return 1;
         // RFC 4231 test vectors
         test(hmac, key1, sizeof(key1), data1, sizeof(data1));
         test(hmac, key2, sizeof(key2), data2, sizeof(data2));