use explicit returns to avoid checker confusion

The checker does not understand the idiom

if (except.reason < 0) {
   do_cleanups (whatever);
   GDB_PY_HANDLE_EXCEPTION (except);
}

because it doesn't realize that the nested 'if' actually has the same
condition.

This fixes instances of this to be more explicit.

	* python/py-breakpoint.c (bppy_get_commands): Use
	explicit, unconditional return.
	* python/py-frame.c (frapy_read_var): Likewise.
	* python/python.c (gdbpy_decode_line): Likewise.

gdb/python/py-frame.c

@@ -477,7 +477,8 @@ frapy_read_var (PyObject *self, PyObject *args)
       if (except.reason < 0)
 	{
 	  do_cleanups (cleanup);
-	  GDB_PY_HANDLE_EXCEPTION (except);
+	  gdbpy_convert_exception (except);
+	  return NULL;
 	}
 
       if (!var)