[gdb/exp] Fix exception when printing optimized out vla

[gdb/exp] Fix exception when printing optimized out vla

When compiling vla-optimized-out.c with -O3 and a recent gcc, and trying to
print the vla a in f1, we run into this gdb exception:
...
Cannot find matching parameter at DW_TAG_call_site 0x4003be at main
...

This is a regression introduced by 42dc7699a2 "[gdb/exp] Fix printing of type
of optimized out vla".

This patch fixes the regression by wrapping the ctx.eval call in
dwarf2_locexpr_baton_eval in try/catch, similar to what is done in
dwarf2_evaluate_loc_desc_full.

Build and reg-tested on x86_64-linux.

2018-07-25  Tom de Vries  <tdevries@suse.de>

	* dwarf2loc.c (dwarf2_locexpr_baton_eval): Wrap ctx.eval call in
	try/catch.

	* gdb.base/vla-optimized-out-o3.exp: New file.  Reuse
	vla-optimized-out.c.