Ensure that LoopEnd > LoopStart

For the benefit of Clang's static analysis, which doesn't know that a
static source will always have a valid buffer and a buffer must have a
non-0 loop range.

Alc/mixer.c

@@ -367,6 +367,7 @@ ALvoid MixSource(ALactivesource *src, ALCdevice *Device, ALuint SamplesToDo)
 
             if(Looping && Source->SourceType == AL_STATIC)
             {
+                assert(LoopEnd > LoopStart);
                 DataPosInt = ((DataPosInt-LoopStart)%(LoopEnd-LoopStart)) + LoopStart;
                 break;
             }