mug: initiliaze Point p

gcc-7 had a complaint.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

sys/src/cmd/mug.c

@@ -1033,7 +1033,7 @@ main(int argc, char **argv)
 	double gammatab[256];
 	Event e;
 	Mouse m;
-	Point lastp, p;
+	Point lastp, p = {};
 	Rectangle nselr, rbig9[9];
 
 	ARGBEGIN{