xfsprogs: libxcmd: don't clobber fs_table on realloc()

xfsprogs: libxcmd: don't clobber fs_table on realloc()

In fs_table_insert(), realloc() is called to resize the global
fs_table.  If it fails, it overwrites a previously valid fs_table
pointer with NULL.

Instead, assign the return value to a local temporary and overwrite
fs_table only if the realloc() call succeeds.  The only defined
errno value for a realloc() failure is ENOMEM, so return that
explicitly in the event it fails.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>