qemu-iotests: Test snapshot=on with nonexistent TMPDIR

We just fixed a bug that was causing a use-after-free when QEMU was
unable to create a temporary snapshot. This is a test case for this
scenario.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/051

@@ -354,6 +354,9 @@ printf %b "qemu-io $device_id \"write -P 0x33 0 4k\"\ncommit $device_id\n" |
 
 $QEMU_IO -c "read -P 0x33 0 4k" "$TEST_IMG" | _filter_qemu_io
 
+# Using snapshot=on with a non-existent TMPDIR
+TMPDIR=/nonexistent run_qemu -drive driver=null-co,snapshot=on
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/051.out

@@ -455,4 +455,7 @@ wrote 4096/4096 bytes at offset 0
 
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Testing: -drive driver=null-co,snapshot=on
+QEMU_PROG: -drive driver=null-co,snapshot=on: Could not get temporary filename: No such file or directory
+
 *** done

tests/qemu-iotests/051.pc.out

@@ -527,4 +527,7 @@ wrote 4096/4096 bytes at offset 0
 
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Testing: -drive driver=null-co,snapshot=on
+QEMU_PROG: -drive driver=null-co,snapshot=on: Could not get temporary filename: No such file or directory
+
 *** done