Fix type mismatch in opus_multistream_decode_native #565

Job	Run time
CMake MINGW	52s
CMake/Android/So/ARMv8/Release	36s
CMake/Android/So/X86/Release	36s
Test build with CMake 3.1.0	1m 10s
CMake/Android/So/X64/Release	41s
CMake/Windows/Dll/X86/Release	1m 38s
CMake/Windows/Dll/X64/Release	2m 21s
CMake/Windows/Dll/ARMv8/Release	1m 48s
CMake/Linux/So/X64/Release	36s
CMake/MacOSX/So/X64/Release	1m 30s
CMake/MacOSX/Framework/X64/Release	1m 18s
CMake/iOS/Dll/arm64/Release	1m 2s
CMake/iOS/Framework/arm64/Release	48s
CMake/Windows/Lib/X86/Release	3m 45s
CMake/Windows/Lib/X64/Release	2m 28s
CMake/Windows/Lib/armv8/Release	1m 39s
CMake/Linux/Lib/X64/Release	1m 1s
CMake/MacOSX/Lib/X64/Release	1m 16s
CMake/iOS/Lib/arm64/Release	1m 12s
CMake/Android/Lib/ARMv8/Release	32s
CMake/Android/Lib/X86/Release	33s
CMake/Android/Lib/X64/Release	42s
CMake/CustomModes/Linux/Lib/X64/Release	1m 10s
CMake/AssertionsFuzz/Windows/Lib/X64/Release	2m 17s
CMake/AssertionsFuzz/Linux/Lib/X64/Release	1m 2s
CMake/AssertionsFuzz/MacOSX/Lib/X64/Release	2m 9s
	34m 42s

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