shared/gatt-client: Fix attempting to discovery with handle 0

If a service was removed an no other service was added it may happen
that pending_svc has not empty but first_svc and last_svc has not been
set causing the following request:

< ACL Data TX: Handle 76 flags 0x00 dlen 11
      ATT: Read By Type Request (0x08) len 6
        Handle range: 0x0000-0x0000
        Attribute type: Include (0x2802)
1 file changed