Silence maybe-uninitialized warning in dwarf2read.c

Silence maybe-uninitialized warning in dwarf2read.c

I upgraded to Fedora 30 recently.  It includes GCC 9, which gives a
warning for dwarf2read.c:

../../binutils-gdb/gdb/dwarf2read.c:16103:24: warning: ‘discr_offset’ may be used uninitialized in this function [-Wmaybe-uninitialized]

This patch fixes the problem by initializing discr_offset.
Tested by rebuilding.

gdb/ChangeLog
2019-12-04  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (process_structure_scope): Initialize
	"discr_offset".

Change-Id: I76a6157921c9beacb641b8a41e10026006621b95