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devdraw: fix topnwindows() panic when images are not windows (thanks aiju)

Crashes drawterm and native:

#include <u.h>
#include <libc.h>
#include <draw.h>

void
main(int argc, char **argv)
{
 	initdraw(nil, nil, nil);

 	Image *a[] = {screen, display->black};

 	topnwindows(a, nelem(a));
 	flushimage(display, 1);
}

--- a/sys/src/9/port/devdraw.c

+++ b/sys/src/9/port/devdraw.c

@@ -2022,13 +2022,13 @@

 				free(lp);

 				nexterror();

 			}

-			for(j=0; j<nw; j++)

+			for(j=0; j<nw; j++){

 				lp[j] = drawimage(client, a+1+1+2+j*4);

-			if(lp[0]->layer == 0)

-				error("images are not windows");

-			for(j=1; j<nw; j++)

+				if(lp[j]->layer == 0)

+					error("images are not windows");

 				if(lp[j]->layer->screen != lp[0]->layer->screen)

 					error("images not on same screen");

+			}

 			if(a[1])

 				memltofrontn(lp, nw);

 			else