Fix memory error for AudioFifo properties

Validate self.ptr before access: uninitialized properties should return None
rather than crash the Python interpreter

Author: zzjjbb

av/audio/fifo.pyx

@@ -176,14 +176,14 @@ cdef class AudioFifo:
     @property
     def format(self):
         """The :class:`.AudioFormat` of this FIFO."""
-        return self.template.format
+        return self.template.format if self.ptr else None
     @property
     def layout(self):
         """The :class:`.AudioLayout` of this FIFO."""
-        return self.template.layout
+        return self.template.layout if self.ptr else None
     @property
     def sample_rate(self):
-        return self.template.sample_rate
+        return self.template.sample_rate if self.ptr else None
 
     @property
     def samples(self):

tests/test_audiofifo.py

@@ -115,3 +115,10 @@ def test_missing_time_base(self) -> None:
         assert oframe is not None
         assert oframe.pts is None and oframe.time_base is None
         assert oframe.sample_rate == iframe.sample_rate
+
+    def test_missing_template(self) -> None:
+        fifo = av.AudioFifo()
+        assert fifo.format is None
+        assert fifo.layout is None
+        assert fifo.sample_rate is None
+        assert fifo.samples == 0