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.

Fixes: d1e9710b63d8 ("scripts/gdb: initial clk support: lx-clk-summary")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 scripts/gdb/linux/clk.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/scripts/gdb/linux/clk.py b/scripts/gdb/linux/clk.py
index 061aecfa294e..7a01fdc3e844 100644
--- a/scripts/gdb/linux/clk.py
+++ b/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@@ -41,6 +41,8 @@ are cached and potentially out of date"""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")
-- 
2.34.1