tools: warn_unused_result build fixes.

tools: warn_unused_result build fixes.

Note that some of the existing error checking for asprintf was wrong
for Linux.  asprintf in glibc returns -1 on error, but leaves the
pointer uninitialized.  Only the BSDs zero out the pointer on error.

Also, while fixing these warnings I saw several error paths that were
incorrect.  This patch minimally fixes the warn_unused_result; more
complete error path cleanup will be a later patch.

Signed-off-by:  Charles Coffing <ccoffing@novell.com>