- fixed: a sight check that starts right on a linedef should ignore that linedef.

Trying to calculate a slope to the target from this linedef's opening will always result in failure because it'd involve a division by zero.

src/p_sight.cpp

@@ -216,6 +216,9 @@ bool SightCheck::PTR_SightTraverse (intercept_t *in)
 	if (open.range == 0 && open.portalflags == 0 && (lport == NULL || lport->mType != PORTT_LINKED))		// quick test for totally closed doors (must be delayed if portal checks are needed, though)
 		return false;		// stop
 
+	if (in->frac == 0)
+		return true;
+
 	// check bottom
 	if (open.bottom > LINEOPEN_MIN)
 	{