mac: fix crash if we can't get an event tap

without assistive-device permissions the event tap can't be create on
10.14 any more which lead to an assertion.

System Preferences > Security & Privacy > Privacy > Accessibility and
add mpv or your terminal App to the list.

(cherry picked from commit ada4f7c600)

osdep/macosx_events.m

@@ -375,6 +375,7 @@ void cocoa_set_mpv_handle(struct mpv_handle *ctx)
 
 - (void)restartMediaKeys
 {
+    if (self->_mk_tap_port)
         CGEventTapEnable(self->_mk_tap_port, true);
 }
 
@@ -410,10 +411,10 @@ void cocoa_set_mpv_handle(struct mpv_handle *ctx)
             tap_event_callback,
             self);
 
-        assert(self->_mk_tap_port != nil);
+        if (self->_mk_tap_port) {
-
             NSMachPort *port = (NSMachPort *)self->_mk_tap_port;
             [[NSRunLoop mainRunLoop] addPort:port forMode:NSRunLoopCommonModes];
+        }
     });
 }
 
@@ -421,10 +422,12 @@ void cocoa_set_mpv_handle(struct mpv_handle *ctx)
 {
     dispatch_async(dispatch_get_main_queue(), ^{
         NSMachPort *port = (NSMachPort *)self->_mk_tap_port;
+        if (port) {
             CGEventTapEnable(self->_mk_tap_port, false);
             [[NSRunLoop mainRunLoop] removePort:port forMode:NSRunLoopCommonModes];
             CFRelease(self->_mk_tap_port);
             self->_mk_tap_port = nil;
+        }
     });
 }