qemu: blockcommit: Fix placement of qemuDomainDiskBlockJobIsSupported

Commit b50a8354 added call to qemuDomainDiskBlockJobIsSupported prior
to filling the 'disk' variable resulting in a crash when attempting a
block commit.

https://gitlab.com/libvirt/libvirt/-/issues/31Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -18504,9 +18504,6 @@ qemuDomainBlockCommit(virDomainPtr dom,
     if (virDomainObjCheckActive(vm) < 0)
         goto endjob;
 
-    if (!qemuDomainDiskBlockJobIsSupported(vm, disk))
-        goto endjob;
-
     blockdev = virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV);
 
     /* Convert bandwidth MiB to bytes, if necessary */
@@ -18523,6 +18520,9 @@ qemuDomainBlockCommit(virDomainPtr dom,
     if (!(disk = qemuDomainDiskByName(vm->def, path)))
         goto endjob;
 
+    if (!qemuDomainDiskBlockJobIsSupported(vm, disk))
+        goto endjob;
+
     if (virStorageSourceIsEmpty(disk->src)) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("disk %s has no source file to be committed"),