[runtime] Be a bit defensive after handling Objective-C exceptions and return a consistent value. (#11601)

The value should never be used if everything is working fine, but if something
is wrong, then this value will at least get consistent behavior.

The visible result for us right now is that monotouch-test will fail with a
test failure instead of crashing.

runtime/trampolines-arm64-objc_msgSend.inc.s

@@ -133,6 +133,11 @@ Lcatchbegin:
 	bl	_xamarin_process_nsexception_using_mode
 Lcatchend:
 	bl	_objc_end_catch
+	; xamarin_process_nsexception_using_mode might have set a pending managed exception, which means we end up executing more code here
+	; in theory, any pending managed exceptions should be thrown automatically upon returning to managed code.
+	; however, things may go wrong, and if the pending managed exception isn't thrown, then make sure we don't return some random value.
+	; returning a constant NULL/0 is at least consistent behavior, and is less likely to crash the process than any other value.
+	mov x0, #0
 	b	Lafterinvoke
 Lcatchcatchhandler:
 	mov	x19, x0