blockjob: avoid memory leak during block pivot

blockjob: avoid memory leak during block pivot

Valgrind caught a memory leak:

==2018== 9 bytes in 1 blocks are definitely lost in loss record 143 of 927
==2018==    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==2018==    by 0x8C42369: strdup (strdup.c:42)
==2018==    by 0x50EACC9: virStrdup (virstring.c:676)
==2018==    by 0x50E79E5: virStorageSourceCopy (virstoragefile.c:1845)
==2018==    by 0x20A3FAA7: qemuDomainBlockCommit (qemu_driver.c:15620)
==2018==    by 0x51DC6B2: virDomainBlockCommit (libvirt.c:20092)

I traced it to the fact that blockcopy and blockcommit end up
reparsing a backing chain on pivot, but the chain parsing code
doesn't gracefully handle the case where the backing file is
already known.

I'm not exactly sure when this was introduced, but suspect that the
refactoring in commit 9944b71 and friends that moved towards probing
in-place rather than into a temporary structure are part of the cause.

* src/util/virstoragefile.c (virStorageFileGetMetadataInternal):
Don't leak any prior value.

Signed-off-by: Eric Blake <eblake@redhat.com>