Avoid crash with absolute symbol

A user supplied an executable and a remote logfile that could be used
to crash gdb.  The problem is that the BFD section for a particular
symbol was null, because the section was not marked "allocated".
Digging deeper, the problem was that elfread.c dropped the section for
absolute symbols.  This patch fixes the crash.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30431