xfsprogs: libxcmd: avoid using strtok()

xfsprogs: libxcmd: avoid using strtok()

The strtok() library routine overwrites delimiting bytes in the
string it is supplied.  It is also not length-constrained.

Since we're making a duplicate of the string anyway, and since we
are only finding the end of a single token, we can do both without
the need to modify the passed-in mount entry structure.

Add checking for memory allocation failures, and if one occurs just
exit (as is the practice elsewhere in this file).

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