rhs may not be defined, say for source_count, so add a guard.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/pmu-events/metric.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/perf/pmu-events/metric.py b/tools/perf/pmu-events/metric.py
index 4797ed4fd817..2f2fd220e843 100644
--- a/tools/perf/pmu-events/metric.py
+++ b/tools/perf/pmu-events/metric.py

@@ -261,8 +261,10 @@ class Function(Expression):
 
   def Equals(self, other: Expression) -> bool:
     if isinstance(other, Function):
-      return self.fn == other.fn and self.lhs.Equals(
-          other.lhs) and self.rhs.Equals(other.rhs)
+      result = self.fn == other.fn and self.lhs.Equals(other.lhs)
+      if self.rhs:
+        result = result and self.rhs.Equals(other.rhs)
+      return result
     return False