tidy: Fix 'value stored is never read' warning in audiooutputbase.cpp.

The clang-tidy "dead stores" check pointed out a variable that is set
and never used, because it is assigned to again before it is
referenced.  Remove the first (dead) assignment and reduce the scope
of the variable to the while loop where it is used.

mythtv/libs/libmyth/audio/audiooutputbase.cpp

@@ -1293,7 +1293,6 @@ bool AudioOutputBase::AddData(void *in_buffer, int in_len,
                               int64_t timecode, int /*in_frames*/)
 {
     int frames   = in_len / m_source_bytes_per_frame;
-    void *buffer = in_buffer;
     int bpf      = m_bytes_per_frame;
     int len      = in_len;
     bool music   = false;
@@ -1340,7 +1339,7 @@ bool AudioOutputBase::AddData(void *in_buffer, int in_len,
         len = m_spdifenc->GetProcessedSize();
         if (len > 0)
         {
-            buffer = in_buffer = m_spdifenc->GetProcessedBuffer();
+            in_buffer = m_spdifenc->GetProcessedBuffer();
             m_spdifenc->Reset();
             frames = len / m_source_bytes_per_frame;
         }
@@ -1418,7 +1417,7 @@ bool AudioOutputBase::AddData(void *in_buffer, int in_len,
 
     while(frames_remaining > 0)
     {
-        buffer = (char *)in_buffer + offset;
+        void *buffer = (char *)in_buffer + offset;
         frames = frames_remaining;
         len = frames * m_source_bytes_per_frame;