Set bounds from the array when iterating mmaps.
The mmap allocation code in vp8_dx_iface.c was inconsistent. The static array vp8_mem_req_segs defines two descriptors, but only the first is real. The second is a sentinel and isn't actually allocated, so vpx_codec_alg_priv is declared with mmaps[NELEMENTS(vp8_mem_req_segs)-1]. Some functions use this reduced upper bound when iterating though the mmap array, but these two functions did not. Instead, this commit calls NELEMENTS(...->mmaps) to directly query the bounds of the dereferenced array. This fixes an array-bounds warning from gcc 4.6 on vp8_xma_set_mmap. Change-Id: I918e2721b401d134c1a9764c978912bdb3188be1