Suppress a memcheck error in Opus decoder

tools/valgrind-webrtc/memcheck/suppressions.txt

 # This file is used in addition to the one already maintained in Chrome.
 # It acts as a place holder for future additions for WebRTC.
 # It must exist for the Python wrapper script to work properly.
 
 # There are three kinds of suppressions in this file.
 # 1. third_party libraries
 # 2. webrtc stuff
 # 3. libjingle stuff (talk folder)
 #-----------------------------------------------------------------------
 
@@ skipping 439 matching lines @@
    fun:_ZN3rtc12_GLOBAL__N_122CreateClientConnectionERNS_10HttpServerERNS0_17HttpServerMonitorEb
    fun:_ZN3rtc47HttpServer_SignalsCloseAfterForcedCloseAll_Test8TestBodyEv
 }
 {
    bug_5989
    Memcheck:Unaddressable
    ...
    fun:_ZNK3rtc15CriticalSection5EnterEv
    ...
 }
+{
+   bug_6444
+   Memcheck:Unaddressable
+   fun:pitch_search
+   fun:run_prefilter
+   fun:celt_encode_with_ec
+   fun:opus_encode_native
+   fun:opus_encode
+   fun:WebRtcOpus_Encode
+   fun:_ZN6webrtc16AudioEncoderOpus10EncodeImplEjN3rtc9ArrayViewIKsEEPNS1_7BufferTIhEE
+   fun:_ZN6webrtc12AudioEncoder6EncodeEjN3rtc9ArrayViewIKsEEPNS1_7BufferTIhEE
+   fun:_ZN6webrtc16AudioDecoderTest11EncodeFrameEPKsmPN3rtc7BufferTIhEE
+   fun:_ZN6webrtc16AudioDecoderTest16EncodeDecodeTestEmidii
+   fun:_ZN6webrtc44AudioDecoderOpusStereoTest_EncodeDecode_Test8TestBodyEv
+}