tests: add check_invalid_maps to test-mmap

This adds a test to make sure we fail properly for a 0 length mmap.
There are most likely other failure conditions we should also check.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: umarcor <1783362@bugs.launchpad.net>
Message-Id: <20180730134321.19898-3-alex.bennee@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>