src: fix up mmap() error checking

src: fix up mmap() error checking

I noticed that in some of my C tests in src/ I was incorrectly
checking for mmap() failure by looking for NULL instead of
MAP_FAILED.  Fix those and clean up some places where we were
testing against -1 (the actual value of MAP_FAILED) which was
manually being cast to a pointer.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>