fix PR symtab/15719

fix PR symtab/15719

This patch fixes PR symtab/15719.

The bug is that "watch -location" crashes on a certain expression.

The problem is that fetch_subexp_value is catching an exception.
For ordinary watchpoints this is ok; but for location watchpoints,
it is better for the exception to propagate.

Built and regtested on x86-64 Fedora 18.
New test case included.

	PR symtab/15719:
	* breakpoint.c (update_watchpoint, watchpoint_check)
	(watch_command_1): Update.
	* eval.c (fetch_subexp_value): Add "preserve_errors"
	parameter.
	* ppc-linux-nat.c (check_condition): Update.
	* value.h (fetch_subexp_value): Update.

	* gdb.base/watchpoint.c (struct foo5): New.
	(nullptr): New global.
	* gdb.base/watchpoint.exp (test_watch_location): Add test.