smoke: fix valgrind invalid read errors

There are a number of invalid read errors reported by valgrind of the
form:
	==13428== Invalid read of size 4
        ==13428==    at 0x405656: advect (smoke.c:116)
        ==13428==    by 0x405E80: redraw_handler (smoke.c:228)
        ==13428==    by 0x40DE74: widget_redraw (window.c:3995)
        ==13428==    by 0x40E02D: surface_redraw (window.c:4053)
        ==13428==    by 0x40E0C9: idle_redraw (window.c:4082)
        ==13428==    by 0x410FC9: display_run (window.c:5561)
        ==13428==    by 0x406518: main (smoke.c:373)
        ==13428==  Address 0xb2c9b14 is 4 bytes after a block of size
                   160,000 alloc'd
        ==13428==    at 0x4C29DB4: calloc
        ==13428==    by 0x40646B: main (smoke.c:360)

This results in invalid rendering when running a debug version of the
application.

Fix the issue by limiting the maximum values of px and py to 1.5 less
than width and height. This prevents reading past the end of the source
buffer.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=82287
Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/smoke.c

@@ -87,10 +87,10 @@ static void advect(struct smoke *smoke, uint32_t time,
 				px = 0.5;
 			if (py < 0.5)
 				py = 0.5;
-			if (px > smoke->width - 0.5)
-				px = smoke->width - 0.5;
-			if (py > smoke->height - 0.5)
-				py = smoke->height - 0.5;
+			if (px > smoke->width - 1.5)
+				px = smoke->width - 1.5;
+			if (py > smoke->height - 1.5)
+				py = smoke->height - 1.5;
 			i = (int) px;
 			j = (int) py;
 			fx = px - i;