vty: fix output of empty IMSI

Check *subscr->imsi, not subscr->imsi, since it is a char[]; same as msisdn
below already does.

Was introduced in change I42b3b70a0439a8f2e4964d7cc31e593c1f0d7537 / commit
183e7009afc8577f0d89c99f92a5720697040494.

Fixes: coverity CID 178166
Change-Id: I72e13efefbac0495b8dd1949a39fa44ebfd46b56
1 file changed