gdb: Improve scoped_switch_fork_info class

After committing this patch I got this feedback:

   https://sourceware.org/ml/gdb-patches/2019-01/msg00181.html

This patch makes the constructor of scoped_switch_fork_info explicit,
and wraps the core of the destructor in a TRY/CATCH block.

I've run this through the testsuite on X86-64/GNU Linux, however, this
code is not exercised, so this patch is untested.

gdb/ChangeLog:

	* linux-fork.c (scoped_switch_fork_info)
	<scoped_switch_fork_info>: Make explicit.
	<~scoped_switch_fork_info>: Wrap core in TRY/CATCH.