Ensure MP3 can safely exit on OOM (#716)

Fixes #715.  Good catch, @EternityForest

Author: earlephilhower

src/AudioGeneratorMP3.cpp

@@ -27,6 +27,9 @@ AudioGeneratorMP3::AudioGeneratorMP3()
   file = NULL;
   output = NULL;
   buff = NULL;
+  synth = NULL;
+  frame = NULL;
+  stream = NULL;
   nsCountMax = 1152/32;
   madInitted = false;
 }
@@ -37,6 +40,9 @@ AudioGeneratorMP3::AudioGeneratorMP3(void *space, int size): preallocateSpace(sp
   file = NULL;
   output = NULL;
   buff = NULL;
+  synth = NULL;
+  frame = NULL;
+  stream = NULL;
   nsCountMax = 1152/32;
   madInitted = false;
 }
@@ -51,6 +57,9 @@ AudioGeneratorMP3::AudioGeneratorMP3(void *buff, int buffSize, void *stream, int
   file = NULL;
   output = NULL;
   buff = NULL;
+  synth = NULL;
+  frame = NULL;
+  stream = NULL;
   nsCountMax = 1152/32;
   madInitted = false;
 }