installers: handle asprintf() correctly
It appears that the glibc version of asprintf() is braindamaged, and
doesn't set the target pointer to NULL in the event of an error (only
returns -1). Therefore we need to check the return value. Just in
case someone else made the *opposite* error, also check the pointer.
Bleh. The glibc documentation states that *BSD sets the pointer to
NULL, but instead of following that, the glibc people put
warn_unused_result on asprintf. Sigh.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>