virDomainObjListAddLocked: fix double free

If @vm has flagged as "to be removed" virDomainObjListFindByNameLocked
returns NULL (although the definition actually exists). Therefore, the
possibility exits that "virHashAddEntry" will raise the error
"Duplicate key" => virDomainObjListAddObjLocked fails =>
virDomainObjEndAPI(&vm) is called and this leads to a freeing of @def
since @def is already assigned to vm->def. But actually this leads to
a double free since the common usage pattern is that the caller of
virDomainObjListAdd(Locked) is responsible for freeing @def in case of
an error.

Let's fix this by setting vm->def to NULL in case of an error.

Backtrace:

   ➤  bt
   #0  virFree (ptrptr=0x7575757575757575)
   #1  0x000003ffb5b25b3e in virDomainResourceDefFree
   #2  0x000003ffb5b37c34 in virDomainDefFree
   #3  0x000003ff9123f734 in qemuDomainDefineXMLFlags
   #4  0x000003ff9123f7f4 in qemuDomainDefineXML
   #5  0x000003ffb5cd2c84 in virDomainDefineXML
   #6  0x000000011745aa82 in remoteDispatchDomainDefineXML
   ...

Reviewed-by: Bjoern Walk <bwalk@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>

src/conf/virdomainobjlist.c

@@ -329,8 +329,10 @@ virDomainObjListAddLocked(virDomainObjListPtr doms,
             goto cleanup;
         vm->def = def;
 
-        if (virDomainObjListAddObjLocked(doms, vm) < 0)
+        if (virDomainObjListAddObjLocked(doms, vm) < 0) {
+            vm->def = NULL;
             goto error;
+        }
     }
  cleanup:
     return vm;