Make sure prerendering doesn't try reaching past last page

Should fix #688

Author: fnevgeny

src/classes/view/pdf.vala

@@ -159,15 +159,25 @@ namespace pdfpc {
             }
 
             int last_slide;
-            if (this.user_slides) {
+            // negative number means prerender all
+            if (Options.prerender_slides < 0) {
+                last_slide = this.n_slides - 1;
+            }
+            else if (this.user_slides) {
                 var user_slide = metadata.real_slide_to_user_slide(this.current_slide_number);
                 var last_user_slide = user_slide + Options.prerender_slides;
-                last_slide = metadata.user_slide_to_real_slide(last_user_slide, true);
+                if (last_user_slide < metadata.get_user_slide_count()) {
+                    last_slide =
+                        metadata.user_slide_to_real_slide(last_user_slide, true);
+                } else {
+                    last_slide = this.n_slides - 1;
+                }
             } else {
-                last_slide = this.current_slide_number + Options.prerender_slides;
-            }
-            if (last_slide >= this.n_slides) {
-                last_slide = this.n_slides - 1;
+                last_slide = this.current_slide_number +
+                    Options.prerender_slides;
+                if (last_slide >= this.n_slides) {
+                    last_slide = this.n_slides - 1;
+                }
             }
 
             int width, height;