Add missing check for a null surface from image::get_image()

Fixes a NULL pointer dereference when drawing preview pane overlays in
the Recall Unit dialog for a unit whose overlays list includes an image
that doesn't exist or can't be read, and debug mode is not enabled.

This didn't happen in debug mode because image::get_image() retrieves a
valid placeholder surface (currently images/misc/missing-image.png) in
that situation with debug mode enabled.

Reported by SkyOne in the forums: http://r.wesnoth.org/t39983

Thanks to mattsc for pinpointing the commit at fault.

Note: the code handling overlays on the listbox/menu of the Recall Unit
dialog shouldn't be affected by this since it uses the BLIT image path
function instead of accessing surfaces directly.

changelog

@@ -55,6 +55,9 @@ Version 1.11.8+dev:
      the game to crash (bug #21351).
    * Changed: Added -Wextra-semi to pedantic compilation.
    * Changed: Added -Wconditional-uninitialized to pedantic compilation.
+   * Fixed NULL pointer dereference when viewing units in the Recall Unit
+     dialog including nonexistent/unreadable images in their overlays, while
+     not in debug mode.
 
 Version 1.11.8:
  * Add-ons client:

src/dialogs.cpp

@@ -1121,6 +1121,10 @@ void unit_preview_pane::draw_contents()
 			BOOST_FOREACH(const std::string& overlay, det.overlays) {
 				surface os = image::get_image(overlay);
 
+				if(!os) {
+					continue;
+				}
+
 				if(os->w > rect.w || os->h > rect.h) {
 					os = scale_surface(os, rect.w, rect.h, false);
 				}