events - Fixed overflow of timeout on STM32F4

For equeue_sema_wait, -1 is used to indicate an infinite wait.
This wasn't handled in the nonrtos implementation and caused
undefined/weird behaviour after an overflow on integer multiplication.

On most boards, the infinite wait would return after ~50 days, on the
STM32F4 the timeout killed all other timeouts for some reason.

equeue_mbed.cpp

@@ -113,7 +113,9 @@ static void equeue_sema_timeout(equeue_sema_t *s) {
 bool equeue_sema_wait(equeue_sema_t *s, int ms) {
     int signal = 0;
     Timeout timeout;
-    timeout.attach_us(s, equeue_sema_timeout, ms*1000);
+    if (ms > 0) {
+        timeout.attach_us(callback(equeue_sema_timeout, s), ms*1000);
+    }
 
     core_util_critical_section_enter();
     while (!*s) {