draw: add badrect() function to reject zero, negative size or orverly huge rectangles

not checking the rectangle dimensions causes integer overflows
and memory corruption. adding a new badrect() function that checks
for these cases.

sys/src/libdraw/badrect.c

@@ -0,0 +1,22 @@
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+
+/*
+ * check for zero, negative size or insanely huge rectangle.
+ */
+int
+badrect(Rectangle r)
+{
+	int x, y;
+	uint z;
+
+	x = Dx(r);
+	y = Dy(r);
+	if(x > 0 && y > 0){
+		z = x*y;
+		if(z/x == y && z < 0x10000000)
+			return 0;
+	}
+	return 1;
+}