Extract emAudioWorkletPostMessage helper. NFC

This avoids duplication and also means we can add consistent checks
on the audio context that is passed in.

Its also a code size win.  We don't have a specific code size test that
references these functions but I ran
`browser.test_audio_worklet_post_function_closure` before and after this
change and got these results:

Before:

13297 out/test/test.js

After:

13152 out/test/test.js

Author: sbc100

src/lib/libwebaudio.js

@@ -403,8 +403,17 @@ var LibraryWebAudio = {
     emscripten_audio_worklet_post_function_1(audioContext, funcPtr, arg0)
   },
 
+  $emAudioWorkletPostMessage__internal: true,
+  $emAudioWorkletPostMessage(audioContext, msg) {
+#if ASSERTIONS || WEBAUDIO_DEBUG
+    if (audioContext) emAudioExpectContex(audioContext, 'emAudioWorkletPostMessage');
+#endif
+    (audioContext ? EmAudio[audioContext].audioWorklet['port'] : port).postMessage(msg);
+  },
+
+  $emscripten_audio_worklet_post_function_2__deps: ['$emAudioWorkletPostMessage'],
   $emscripten_audio_worklet_post_function_2: (audioContext, funcPtr, arg0, arg1) => {
-    (audioContext ? EmAudio[audioContext].audioWorklet['port'] : port).postMessage({'_wsc': funcPtr, args: [arg0, arg1] }); // "WaSm Call"
+    emAudioWorkletPostMessage(audioContext, {'_wsc': funcPtr, args: [arg0, arg1] }); // "WaSm Call"
   },
 
   emscripten_audio_worklet_post_function_vii__deps: ['$emscripten_audio_worklet_post_function_2'],
@@ -417,8 +426,9 @@ var LibraryWebAudio = {
     emscripten_audio_worklet_post_function_2(audioContext, funcPtr, arg0, arg1);
   },
 
+  $emscripten_audio_worklet_post_function_3__deps: ['$emAudioWorkletPostMessage'],
   $emscripten_audio_worklet_post_function_3: (audioContext, funcPtr, arg0, arg1, arg2) => {
-    (audioContext ? EmAudio[audioContext].audioWorklet['port'] : port).postMessage({'_wsc': funcPtr, args: [arg0, arg1, arg2] }); // "WaSm Call"
+    emAudioWorkletPostMessage(audioContext, {'_wsc': funcPtr, args: [arg0, arg1, arg2] }); // "WaSm Call"
   },
   emscripten_audio_worklet_post_function_viii__deps: ['$emscripten_audio_worklet_post_function_3'],
   emscripten_audio_worklet_post_function_viii: (audioContext, funcPtr, arg0, arg1, arg2) => {
@@ -429,16 +439,15 @@ var LibraryWebAudio = {
     emscripten_audio_worklet_post_function_3(audioContext, funcPtr, arg0, arg1, arg2);
   },
 
-  emscripten_audio_worklet_post_function_sig__deps: ['$readEmAsmArgs'],
+  emscripten_audio_worklet_post_function_sig__deps: ['$readEmAsmArgs', '$emAudioWorkletPostMessage'],
   emscripten_audio_worklet_post_function_sig: (audioContext, funcPtr, sigPtr, varargs) => {
 #if ASSERTIONS
-    assert(audioContext >= 0);
     assert(funcPtr);
     assert(sigPtr);
     assert(UTF8ToString(sigPtr)[0] != 'v', 'Do NOT specify the return argument in the signature string for a call to emscripten_audio_worklet_post_function_sig(), just pass the function arguments.');
     assert(varargs);
 #endif
-    (audioContext ? EmAudio[audioContext].audioWorklet['port'] : port).postMessage({'_wsc': funcPtr, args: readEmAsmArgs(sigPtr, varargs) });
+    emAudioWorkletPostMessage(audioContext, {'_wsc': funcPtr, args: readEmAsmArgs(sigPtr, varargs)});
   }
 };