gdb: add asserts in thread code

Unlike the previous patch, I don't propose that we take this patch into
gdb-10-branch.

This patch adds two asserts, prompted by investigating and fixing the
bug fixed by the previous patch.

The assert in find_thread_ptid would have caught the original issue
before the segfault (I think it's slightly more use friendly).

The assert in add_thread_silent would have made it clear that the
solution proposed in [1] isn't the right one.  The solution ended up
passing nullptr as a target to add_thread.  We don't want that, because
add_thread_silent uses it to look up the inferior to which to add the
thread.  If the target is nullptr, we could find an inferior with the
same pid, but belonging to an unrelated target.  So we always want a
non-nullptr target in add_thread_silent.

gdb/ChangeLog:

	* thread.c (add_thread_silent): Add assert.
	(find_thread_ptid): Add assert.

[1] https://sourceware.org/pipermail/gdb-patches/2021-February/176202.html

Change-Id: Ie593ee45c5eb02235e8e9fbcda612d48ce883852

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