qemu: fix a crash when save file can't be opened

In qemuDomainSaveMemory, wrapperFd might be NULL and should be checked before
calling virFileWrapperFdCatchError. Same in doCoreDump.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=880919

src/qemu/qemu_driver.c

@@ -2906,7 +2906,8 @@ qemuDomainSaveMemory(struct qemud_driver *driver,
 
 cleanup:
     VIR_FORCE_CLOSE(fd);
-    virFileWrapperFdCatchError(wrapperFd);
+    if (wrapperFd)
+        virFileWrapperFdCatchError(wrapperFd);
     virFileWrapperFdFree(wrapperFd);
     VIR_FREE(xml);
 
@@ -3362,7 +3363,8 @@ doCoreDump(struct qemud_driver *driver,
 cleanup:
     VIR_FORCE_CLOSE(fd);
     if (ret != 0) {
-        virFileWrapperFdCatchError(wrapperFd);
+        if (wrapperFd)
+            virFileWrapperFdCatchError(wrapperFd);
         unlink(path);
     }
     virFileWrapperFdFree(wrapperFd);