breakpoints/19546: Fix crash after updating breakpoints

One of the last checks update_breakpoints_after_exec does while looping
over the list of breakpoints is check that the breakpoint has a valid
location spec. It uses event_location_empty_p to check if the location spec
is "empty", and if it is, the breakpoint is deleted.

momentary_breakpoint types rely on setting the breakpoint structure's
location spec to NULL, thereby causing an update to delete the breakpoint.
However, event_location_empty_p assumed that locations were never NULL.
As a result, GDB would crash dereferencing a NULL pointer whenever
update_breakpoints_after_exec would encounter a momentary_breakpoint.

This patch creates a new wrapper/helper function which tests that the given
breakpoint's location spec is non-NULL and if it is not "empty"
or "unspecified."

gdb/ChangeLog

	PR breakpoints/19546
	* breakpoint.c (breakpoint_event_location_empty_p): New function.
	(update_breakpoints_after_exec, bkpt_re_set): Use this new function
	instead of event_location_empty_p.

gdb/testsuite/ChangeLog

	PR breakpoints/19546
	* gdb.base/infcall-exec.c: New file.
	* gdb.base/infcall-exec2.c: New file.
	* gdb.base/infcall-exec.exp: New file.