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>