Don't try using the DirectSound backend if not all APIs are available.

This affects Windows 95. It does not appear to have at least one of
these functions. This is convenient because the function
IDirectSound::CreateSoundBuffer() is failing in my tests and I wasn't
able to figure out what was going on. Simply failing context
initialization outright is an acceptable solution for now since
Windows 95 is such an uncommon target and it can always fall back to
WinMM which works.

miniaudio.h

@@ -25010,6 +25010,18 @@ static ma_result ma_context_init__dsound(ma_context* pContext, const ma_context_
     pContext->dsound.DirectSoundCaptureCreate     = ma_dlsym(pContext, pContext->dsound.hDSoundDLL, "DirectSoundCaptureCreate");
     pContext->dsound.DirectSoundCaptureEnumerateA = ma_dlsym(pContext, pContext->dsound.hDSoundDLL, "DirectSoundCaptureEnumerateA");
 
+    /*
+    We need to support all functions or nothing. DirectSound with Windows 95 seems to not work too
+    well in my testing. For example, it's missing DirectSoundCaptureEnumerateA(). This is a convenient
+    place to just disable the DirectSound backend for Windows 95.
+    */
+    if (pContext->dsound.DirectSoundCreate            == NULL ||
+        pContext->dsound.DirectSoundEnumerateA        == NULL ||
+        pContext->dsound.DirectSoundCaptureCreate     == NULL ||
+        pContext->dsound.DirectSoundCaptureEnumerateA == NULL) {
+        return MA_API_NOT_FOUND;
+    }
+
     pCallbacks->onContextInit             = ma_context_init__dsound;
     pCallbacks->onContextUninit           = ma_context_uninit__dsound;
     pCallbacks->onContextEnumerateDevices = ma_context_enumerate_devices__dsound;