Predict integer overflow to avoid buffer overruns.

Several functions, mostly type input functions, calculated an allocation
size such that the calculation wrapped to a small positive value when
arguments implied a sufficiently-large requirement.  Writes past the end
of the inadvertent small allocation followed shortly thereafter.
Coverity identified the path_in() vulnerability; code inspection led to
the rest.  In passing, add check_stack_depth() to prevent stack overflow
in related functions.

Back-patch to 8.4 (all supported versions).  The non-comment hstore
changes touch code that did not exist in 8.4, so that part stops at 9.0.

Noah Misch and Heikki Linnakangas, reviewed by Tom Lane.

Security: CVE-2014-0064
lseg_send(PG_FUNCTION_ARGS)
{
	LSEG	   *ls = PG_GETARG_LSEG_P(0);
	StringInfoData buf;

	pq_begintypsend(&buf);
	pq_sendfloat8(&buf, ls->p[0].x);
	pq_sendfloat8(&buf, ls->p[0].y);
	pq_sendfloat8(&buf, ls->p[1].x);
	pq_sendfloat8(&buf, ls->p[1].y);
	PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}

lseg_send(PG_FUNCTION_ARGS)
{
	LSEG	   *ls = PG_GETARG_LSEG_P(0);
	StringInfoData buf;

	pq_begintypsend(&buf);
	pq_sendfloat8(&buf, ls->p[0].x);
	pq_sendfloat8(&buf, ls->p[0].y);
	pq_sendfloat8(&buf, ls->p[1].x);
	pq_sendfloat8(&buf, ls->p[1].y);
	PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}
