mythzmserver: Add a proper check for a failed call to mmap()

Should be checking for MAP_FAILED not NULL. Also add some additional debugging
showing why we failed to open the mmap file. Fixes #11187.

(cherry picked from commit 52b277397431dd88a337e46a7d64fea89c8037c0)

Signed-off-by: Paul Harrison <pharrison@mythtv.org>