Addressing audio plugin crash

Stereo buffer is not remade in the case of audio plugin
switch (only affects GBA, not GB).  This results in a crash.
We need to remake the stereo buffer and reinit the pointers
in Gba_Pcm.output or they will be dangling, and lead to a
crash.  Also, cleaning up the Multi_Buffer class a bit.

src/apu/Multi_Buffer.h

@@ -58,10 +58,12 @@ class Multi_Buffer
         }
 
         // See Blip_Buffer.h
-        virtual long read_samples(blip_sample_t *, long)
-            BLARGG_PURE({ return 0; }) virtual long samples_avail() const BLARGG_PURE({ return 0; })
+        virtual long read_samples(blip_sample_t *, long) BLARGG_PURE({ return 0; })
+        virtual long samples_avail() const BLARGG_PURE({ return 0; })
 
-                public : BLARGG_DISABLE_NOTHROW void disable_immediate_removal()
+        BLARGG_DISABLE_NOTHROW
+
+        void disable_immediate_removal()
         {
                 immediate_removal_ = false;
         }

src/gba/Sound.cpp

@@ -540,6 +540,10 @@ bool soundInit()
     if (!soundDriver->init(soundSampleRate))
         return false;
 
+    if (!stereo_buffer) {
+        remake_stereo_buffer();
+    }
+
     soundPaused = true;
     return true;
 }