libstagefright: Fix out of bounds error for H264 software decoder

If decoded clip's level is 1.0, InitDPB() and AVCConfigureSequence()
compute different number of frames (num_fs) to allocate.  InitDPB()
holds an extra frame if num_fs is 1, where as AVCCS() does not. Since
InitDPB() sets the state and AVCCS() allocates the buffers, num_fs - 1
buffers will be allocated.  During decode, we'll attempt to access the
num_fs-th frame which will be out of bounds.

This fix sets AVCCS()'s copy of num_fs to be same as whatever InitDPB()
computes.

Change-Id: I4bb5afdb4750084f0737878445f59842cd22a6f7
CRs-Fixed: 276145