Bug 1587244 - Allow NaN in IIR filter output. r=karlt

It's perfectly possible to create unstable filter, that can easily go to
infinity, so this assert is not valid.

Differential Revision: https://phabricator.services.mozilla.com/D49608

--HG--
extra : moz-landing-system : lando

dom/media/webaudio/blink/IIRFilter.cpp

@@ -106,8 +106,8 @@ void IIRFilter::process(const float* sourceP, float* destP,
 
     // Avoid introducing a stream of subnormals
     destP[n] = WebCore::DenormalDisabler::flushDenormalFloatToZero(yn);
-    MOZ_ASSERT(destP[n] == 0.0 || fabs(destP[n]) > FLT_MIN,
-               "output should not be subnormal");
+    MOZ_ASSERT(destP[n] == 0.0 || fabs(destP[n]) > FLT_MIN || IsNaN(destP[n]),
+               "output should not be subnormal, but can be NaN");
   }
 }