do_powerup_frame: reject invalid vclip frametime

vclip::frame_time equal to 0 would cause an infinite loop.
vclip::frame_time less than 0 would cause the loop to run until
underflow, which could be very slow if frame_time is near 0.

similar/main/powerup.cpp

@@ -81,6 +81,8 @@ void do_powerup_frame(const vmobjptridx_t obj)
 	
 	auto &vc = Vclip[vci->vclip_num];
 	const auto vc_frame_time = vc.frame_time;
+	if (vc_frame_time > 0)
+	{
 	const auto vc_num_frames1 = vc.num_frames - 1;
 	vci->frametime -= FrameTime+fudge;
 	
@@ -105,6 +107,7 @@ void do_powerup_frame(const vmobjptridx_t obj)
 				vci->framenum++;
 		}
 	}
+	}
 
 	if (obj->lifeleft <= 0) {
 		object_create_explosion(vmsegptridx(obj->segnum), obj->pos, F1_0*7/2, VCLIP_POWERUP_DISAPPEARANCE);