Fix DECRBY LLONG_MIN caused an overflow (#1581)

Note that this may break compatibility since in the past
doing: `DECRBY key -9223372036854775808` would succeed
but create an invalid result. And now we will return an
error rejecting the request.

Before:
```
127.0.0.1:6666> set key 0
OK
127.0.0.1:6666> decrby key -9223372036854775807
(integer) 9223372036854775807
127.0.0.1:6666> get key
"9223372036854775807"

127.0.0.1:6666> set key 0
OK
127.0.0.1:6666> decrby key -9223372036854775808
(integer) -9223372036854775808
127.0.0.1:6666> get key
"-9223372036854775808"
```

After:
```
127.0.0.1:6666> decrby key -9223372036854775808
(error) ERR decrement would overflow
```

src/commands/cmd_string.cc

@@ -530,6 +530,12 @@ class CommandDecrBy : public Commander {
     }
 
     increment_ = *parse_result;
+
+    // Negating LLONG_MIN will cause an overflow.
+    if (increment_ == LLONG_MIN) {
+      return {Status::RedisParseErr, "decrement would overflow"};
+    }
+
     return Commander::Parse(args);
   }
 

tests/gocase/unit/type/incr/incr_test.go

@@ -84,6 +84,11 @@ func TestIncr(t *testing.T) {
 		require.EqualValues(t, -1, rdb.DecrBy(ctx, "novar", 17179869185).Val())
 	})
 
+	t.Run("DECRBY negation overflow", func(t *testing.T) {
+		require.NoError(t, rdb.Do(ctx, "SET", "foo", "0").Err())
+		util.ErrorRegexp(t, rdb.Do(ctx, "DECRBY", "foo", "-9223372036854775808").Err(), ".*decrement would overflow.*")
+	})
+
 	t.Run("INCRBYFLOAT against non existing key", func(t *testing.T) {
 		require.NoError(t, rdb.Del(ctx, "novar").Err())
 		require.EqualValues(t, 1, rdb.IncrByFloat(ctx, "novar", 1.0).Val())