Avoid calling fclose(...) with an invalid argument

src/tests.c

@@ -4924,8 +4924,10 @@ int main(int argc, char **argv) {
             seed16[6] ^= t >> 48;
             seed16[7] ^= t >> 56;
         }
+        if (frand) {
             fclose(frand);
         }
+    }
     secp256k1_rand_seed(seed16);
 
     printf("test count = %i\n", count);