scripts/gdb: bail early if there are no clocks

Avoid generating an exception if there are no clocks registered:

(gdb) lx-clk-summary
                                 enable  prepare  protect
   clock                          count    count    count        rate
------------------------------------------------------------------------
Python Exception <class 'gdb.error'>: No symbol "clk_root_list" in
current context.
Error occurred in Python: No symbol "clk_root_list" in current context.

Link: https://lkml.kernel.org/r/20230323225246.3302977-1-f.fainelli@gmail.com
Fixes: d1e9710b ("scripts/gdb: initial clk support: lx-clk-summary")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

scripts/gdb/linux/clk.py

@@ -41,6 +41,8 @@ are cached and potentially out of date"""
             self.show_subtree(child, level + 1)
 
     def invoke(self, arg, from_tty):
+        if utils.gdb_eval_or_none("clk_root_list") is None:
+            raise gdb.GdbError("No clocks registered")
         gdb.write("                                 enable  prepare  protect               \n")
         gdb.write("   clock                          count    count    count        rate   \n")
         gdb.write("------------------------------------------------------------------------\n")