vlr_sgs.c: Fix missing use_count decrease in vlr_sgs_imsi_detach

it was recently observed in a pcap trace with gsmtap_log that the
use_count contained a "vlr_sgs_imsi_detach" item despite no related
message was seen near by. Further investigation shows that there's an
unbalanced get+put code path, introduced by an early return added to fix
another issue.

related: SYS#5337
Fixes: 0803d88d9aa6ff36896fbde218018db3bea61dc7
Change-Id: I91ae956e50fca2f4d0e1d145d60ccb0ebfb409e9
1 file changed