gems: prevent invalid Nitpicker_buffer objects

This patch ensures that we never request a zero-sized virtual
framebuffer from nitpicker even when instantiating the object with zero
width or height. It therebu removes the burden of handling the resulting
invalid framebuffer dataspace from the user of the Nitpicker_buffer
utility.

repos/gems/include/gems/nitpicker_buffer.h

@@ -86,7 +86,8 @@ struct Nitpicker_buffer
 	                 Genode::Ram_session &ram)
 	:
 		ram(ram), nitpicker(nitpicker),
-		mode(size.w(), size.h(), nitpicker.mode().format())
+		mode(Genode::max(1UL, size.w()), Genode::max(1UL, size.h()),
+		     nitpicker.mode().format())
 	{ }
 
 	/**