gdb: warn unused result for bfd IO functions

gdb: warn unused result for bfd IO functions

This fixes the compilation warnings introduced by my bfdio.c patch.

The removed bfd_seeks in coff_symfile_read date back to 1994, commit
7f4c859520, prior to which the file used stdio rather than bfd to read
symbols.  Since it now uses bfd to read the file there should be no
need to synchronise to bfd's idea of the file position.  I also fixed
a potential uninitialised memory access.

Approved-By: Andrew Burgess <aburgess@redhat.com>