lib/: exit_if_null(): Add macro to exit(3) on error

lib/: exit_if_null(): Add macro to exit(3) on error

Writing an x*() variant function of several functions is unnecessary.
It's simpler to write a generic exit_if_null() macro that can be chained
with any other calls.  With such a macro, the x*() variants can be
implemented as one-liner macros that are much easier to read:

For example:

	#define xmalloc(size)  exit_if_null(malloc(size))

If an error is detected, log an error, and exit(13).  About why 13, I
don't really know.  It's just what was used previously in xmalloc().

Signed-off-by: Alejandro Colomar <alx@kernel.org>