Check return value of bfd_init

Check return value of bfd_init

Alan recently added a way for BFD library users to check whether they
were in fact loading a compatible version of BFD:

https://sourceware.org/ml/binutils/2018-10/msg00198.html

It seemed reasonable to me that gdb should do this check as well, in
case someone is dynamically linking against BFD.

Simon pointed out that an earlier version of the patch would cause a
gdb crash if the test failed.  This version works around this by
lowering the call to bfd_init and adding a comment explaining where
'error' can safely be called in captured_main_1.

gdb/ChangeLog
2018-10-30  Tom Tromey  <tom@tromey.com>

	* main.c (captured_main_1): Check return value of bfd_init.