Fix screenshot path defaulting to the root directory for unsaved files

Using a "//" prefix resolves to the root directory which isn't a good
default as it typically doesn't have write permissions.
Only set the name and let the file selector pick a directory to use
(matches how saving from the text editor works).

source/blender/editors/screen/screendump.c

@@ -167,12 +167,15 @@ static int screenshot_invoke(bContext *C, wmOperator *op, const wmEvent *event)
 
     /* extension is added by 'screenshot_check' after */
     char filepath[FILE_MAX];
-    BLI_snprintf(filepath, FILE_MAX, "//%s", DATA_("screen"));
     const char *blendfile_path = BKE_main_blendfile_path_from_global();
     if (blendfile_path[0] != '\0') {
       BLI_strncpy(filepath, blendfile_path, sizeof(filepath));
       BLI_path_extension_replace(filepath, sizeof(filepath), ""); /* strip '.blend' */
     }
+    else {
+      /* As the file isn't saved, only set the name and let the file selector pick a directory. */
+      STRNCPY(filepath, DATA_("screen"));
+    }
     RNA_string_set(op->ptr, "filepath", filepath);
 
     WM_event_add_fileselect(C, op);