modpost: fix asprintf warnings
On compilers with security warnings enabled by default, we get:

scripts/mod/modpost.c: In function 'get_markers':
scripts/mod/modpost.c:1562: warning: ignoring return value of 'asprintf',
declared with attribute warn_unused_result
scripts/mod/modpost.c: In function 'add_marker':
scripts/mod/modpost.c:1982: warning: ignoring return value of 'asprintf',
declared with attribute warn_unused_result

So check the return value and abort on errors.

Signed-off-by: Mike Frysinger <vapier@xxxxxxxxxx>
Signed-off-by: garwedgess <garethwilliams21@gmail.com>