core_unwind: Make sure there's always a file_name

Otherwise the server validation fails. Use module name if there's
no canonical path as is the case for linux-gate.so.1 without kernel
debuginfo installed after 3.16 x86 VDSO rework.

Closes abrt/abrt#826

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>