Prevent call to OwnerLostFocusHandler for disposed object

bugzilla.xamarin.com bug reference: Bug 19304
ComboBox reverts to original selection when it loses focus.
The LostFocus handler for the internal ComboTextBox class
was being called after it had been disposed.  The dispose
was correctly called when the ComboBoxStyle changed from
it's initial default value to DropDownList.  However, when
the ComboBox control lost focus later on, the LostFocus event handler
was called, which resulted in the Text value for the ComboBox
being overwritten by the value in the disposed object.

Could not see how to write a unit test within the ComboBoxTest.cs
that would simulate losing focus so only providing fix.

Change-Id: I9ca4cc75da5711a7d7c1dd166d5f52d580f44df1
[runtime] Fix a warning which happens because we store the thread tid as a guint64.
[runtime] Fix some automake conditionals in scripts/, they should be in the beginning of the line.
[jit] Register the Mono_UnhandledException_internal icall when TARGET_ANDROID is set.
[System] Fix build warnings
System: Fix the previous bulid
[corlib] Remove stale comments
[mcs] Convert extended underlying enum constants to their underlying type. Fixes #18866
[counters] Add sample and accessors functions
[counters] Sample should fail if buffer size is too small
[mcs] Emit all method like members in same order as they appear in source code. Fixes #19509
[counters] Use macro to copy counter in sampling, and use sizeof directly instead of mono_counter_get_size

The use of sizeof will enable compilers optimization on memcpy.
[counters] Fix type issue on mini counters

Used MONO_COUNTER_WORD for gint32 variable, crashed on 64bits platform.
[dlr] Emit correct code for convert of nullable types. Fixes #19500
[counters] Disable register filter on valid_mask

This was previously used to not store counters that were not going to be dumped, but because we can now sample them, we have to make sure that they are stored, even if we do not plan to dump or sample them.
[mono-api-html] Report [Obsolete] members in a separate section (not as both removed and added with an [Obsolete]). That makes it easier to read long API diff reports
[Mono.Debugger.Soft] Modified TypeMirror.NewInstance() to check that the method is a .ctor
[sdb] Add a new TypeMirror.NewInstance () method which can create valuetypes without a ctor.
[mono-api-html] Make the [Obsolete] a bit prettier when no text message is available
[mono-api-html] Prettier output of ref and out (instead of &)