timers: handle signed int32 overflow in enroll()

Before this patch calling `socket.setTimeout(0xffffffff)` will result in
signed int32 overflow in C++ which resulted in assertion error:

    Assertion failed: (timeout >= -1), function uv__io_poll, file
    ../deps/uv/src/unix/kqueue.c, line 121.

see #5101

/cc @bnoordhuis