stagefright aacenc: Safeguard against overwriting bits

Previously, if bits above the lowest noBitsToWrite were
set, they would be ORed into the previous cache word,
setting unrelated bits erroneously.

This doesn't noticeably affect the performance of the
codec as a whole.

Change-Id: Ie9935533c4299b8f07cb14485f039a9be9c84016

Author: mstorsjo

media/libstagefright/codecs/aacenc/src/bitbuffer.c

@@ -152,6 +152,7 @@ Word16 WriteBits(HANDLE_BIT_BUF hBitBuf,
 
   wBitPos = hBitBuf->wBitPos;
   wBitPos += noBitsToWrite;
+  writeValue &= ~(0xffffffff << noBitsToWrite); // Mask out everything except the lowest noBitsToWrite bits
   writeValue <<= 32 - wBitPos;
   writeValue |= hBitBuf->cache;