Fix int overflow for 0 deadline timers

Turns out, HAPPlatformTimerRegister may be invoked with deadline of 0 sometimes.
This translates into a negative duration for mgos_set_timer, which is fine
for the first 24.85 days but then duration rolls over and things go south.

Instead, clamp negative duration to 0 while it's still a 64-bit int.

apple/HomeKitADK#102

Fixes mongoose-os-apps/shelly-homekit#129

src/PAL/HAPPlatformTimer.c

@@ -41,6 +41,7 @@ HAPError HAPPlatformTimerRegister(
         return kHAPError_OutOfResources;
     }
     int64_t duration = ((int64_t) deadline - (int64_t) HAPPlatformClockGetCurrent());
+    if (duration < 0) duration = 0;
     ctx->timer_id = mgos_set_timer((int) duration, 0, hap_timer_cb, ctx);
     if (ctx->timer_id == MGOS_INVALID_TIMER_ID) {
         return kHAPError_OutOfResources;