Changes

pcap-server: Validate rolling over file was successful before writing to it

If reopening the file fails (eg. disk full), we'd end up with a null
conn->wrf pointer there which would segfault in
osmo_pcap_wr_file_write_msgb() immediatelly below.

Change-Id: I103d3f66bf40c68db1a20a987bc984989c91b447