misc: Fix a small pile of warnings found with diagnostics turned way up

* possible null pointer dereferences
 * non-annotated fallthroughs in case statements

Change-Id: I2332e0e0a4203ceca0999291ead112b8ab94c613

apps/plugins/goban/sgf.c

@@ -1304,7 +1304,7 @@ read_comment_sgf (char *buffer, size_t buffer_size)
             {
                 break;
             }
-
+            /* Intentional fallthrough */
         default:
             *buffer = temp;
             ++buffer;