BUG/MINOR: guid: fix crash on invalid guid name

BUG/MINOR: guid: fix crash on invalid guid name

Using an invalid GUID for guid_insert() causes a crash. This is easily
reproducible using for example an invalid character with "guid" keyword.
Here is the provided backtrace :

  Thread 1 "haproxy" received signal SIGSEGV, Segmentation fault.
  0x00005555561fda95 in guid_insert (objt=0x520000002080, uid=0x519000002dac "@foo2", errmsg=0x7ffff4c0a7a0)
      at src/guid.c:83
  83              ha_free(&guid->node.key);

This error is present in guid_insert() cleanup parts. GUID node is not
allocated in case of an early error so it's impossible to dereference it
to free guid.node.key. Fix this simply by using an intermediary pointer
key.

This does not need to be backported.