Clip signal at +/-4.0f

Currently we shut out defunct signal values like inf/nan but we apart
from that we allow signals of any size. This will let large but short
shot noise through that can trigger especially reverb to dangerously
large sound levels even though they would pass largely unnoticed
without a feedback loop.
After testing for inf/nan we clamp the sound to +/-4.0f .

src/core/MixHelpers.cpp

@@ -82,6 +82,10 @@ bool sanitize( sampleFrame * src, int frames )
 				src[f][c] = 0.0f;
 				found = true;
 			}
+			else
+			{
+				src[f][c] = qBound( -4.0f, src[f][c], 4.0f );
+			}
 		}
 	}
 	return found;