In late beta and later, `xul.dll` does notexport the check function, and
`firefox.exe` does not try to load it. Unfortunately, a partial update
can result in these two being mismatched -- and does so often enough to
be a beta topcrash.
Suppress this crash by checking whether the diagnostic function is
actually exported and found, rather than asserting that it must be.
(This patch is explicitly just a bandaid, and doesn't address the
underlying issue of the two binaries being mismatching versions.)
Differential Revision: https://phabricator.services.mozilla.com/D218798
8bb6f203e5