stagefright aacenc: Fix reading out of bounds in pow2_xy

This fixes cases where x was a large number, causing fPart to
exceed the 32 bit signed integer range (while fitting in an
unsigned 32 bit integer), making the table index a negative
number.

Change-Id: I674047db65f89148a93d218c138b42cd8305f80e

Author: mstorsjo

media/libstagefright/codecs/aacenc/basic_op/oper_32b.c

@@ -344,8 +344,8 @@ static const Word32 pow2Table[POW2_TABLE_SIZE] = {
 */
 Word32 pow2_xy(Word32 x, Word32 y)
 {
-  Word32 iPart;
-  Word32 fPart;
+  UWord32 iPart;
+  UWord32 fPart;
   Word32 res;
   Word32 tmp, tmp2;
   Word32 shift, shift2;