PR30828, notes obstack memory corruption

PR30828, notes obstack memory corruption

Commit 3bab069c29b3 carelessly allowed "string" to be released from
the notes obstack twice, with the second call to obstack_free
releasing memory for a fixup that just happened to be the same size as
the original string.  The fixup then of course was overwritten.
This patch fixes that problem, and another that could occur on an
error path.

	PR 30828
	* stabs.c (s_stab_generic): Don't free string twice.  Don't
	blow away entire notes obstack on a missing string.