Add missing "return" statement to accumulate_append_subpath.

Without this, Parallel Append can end up with extra children.

Report by Rajkumar Raghuwanshi.  Fix by Amit Khandekar.  Brown
paper bag bug by me.

Discussion: http://postgr.es/m/CAKcux6mBF-NiddyEe9LwymoUC5+wh8bQJ=uk2gGkOE+L8cv=LA@mail.gmail.com

src/backend/optimizer/path/allpaths.c

@@ -1926,6 +1926,7 @@ accumulate_append_subpath(Path *path, List **subpaths, List **special_subpaths)
 							  apath->first_partial_path);
 			*special_subpaths = list_concat(*special_subpaths,
 											new_special_subpaths);
+			return;
 		}
 	}
 	else if (IsA(path, MergeAppendPath))