LibAudio: Initialize m_stream before parsing a FLAC header
Before this change opening the file in the system resulted in crash
caused by assertion saying:
SoundPlayer(32:32): ASSERTION FAILED: m_ptr
../.././AK/OwnPtr.h:139
[#0 SoundPlayer(32:32)]: Terminating SoundPlayer(32) due to signal 6
[#0 FinalizerTask(4:4)]: 0xdeadc0de
The issue was that 845d403b8c175ff99793239404ca8657d95da104 started
using m_stream in the parse_header() function, but that variable wasn't
initialized if the Loader plugin was created using a file path
(which is used everywhere except for the fuzz testing),
resulting in a crash mentioned above.