When assert() is called, which is not uncommon in utility code within NetSurf,
we lose coverage data for anything done before the assert() in the test. This
commit corrects that oversight but is at least slightly GCC specific and may
need tweaks for non-Linux platforms.
By default, 'make coverage' will enable assert coverage, and it can be disabled
with 'make coverage NOASSERTCOVERAGE=yes' if necessary.
Signed-off-by: Daniel Silverstone <dsilvers@netsurf-browser.org>
