Don't try to open the mic device every time if SDL says it has none

Fixes the UI hanging up on Windows 11 when there are no mics, but the mic
input is set to external device as it is by default.

src/frontend/qt_sdl/AudioInOut.cpp

@@ -126,6 +126,10 @@ void MicOpen()
         return;
     }
 
+    int numMics = SDL_GetNumAudioDevices(1);
+    if (numMics == 0)
+        return;
+
     SDL_AudioSpec whatIwant, whatIget;
     memset(&whatIwant, 0, sizeof(SDL_AudioSpec));
     whatIwant.freq = 44100;