kbuild: check return value of asprintf()

Check return value of asprintf() in docsect() and exit if error
occurs. This removes following warning:

  HOSTCC  scripts/basic/docproc
scripts/basic/docproc.c: In function ‘docsect’:
scripts/basic/docproc.c:336: warning: ignoring return value of ‘asprintf’,
				declared with attribute warn_unused_result

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>