gdb/dwarf2read.c: Sanity check DW_AT_sibling values.

gdb/dwarf2read.c: Sanity check DW_AT_sibling values.

When reading objects with corrupt debug information it is possible that
the sibling chain can form a loop, which leads to an infinite loop and
memory exhaustion.

Avoid this situation by disregarding and DW_AT_sibling values that point
to a lower address than the current entry.

gdb/ChangeLog:

2013-11-06  Will Newton  <will.newton@linaro.org>

	PR gdb/12866
	* dwarf2read.c (skip_one_die): Sanity check DW_AT_sibling
	values.  (read_partial_die): Likewise.