check return code of asprintf

On ubuntu (gcc 4.9.2), I get:

mount.c: In function ‘add_fsname_auto’:
mount.c:1414:3: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
   asprintf(&fsauto_names, "%s,%s", old, names);
   ^
cc1: all warnings being treated as errors

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>