Fix ptr overflow warning in filter_ar.c
Fix ptr overflow warning in filter_ar.c

In this code, the problem was that the ptr could sometimes point
outside of the allocated arrays, in particular before the array,
causing a pointer overflow warning. However, the memory pointed to was
never read or written while the pointer was off.

With this change, we keep an index instead of a pointer, which avoids
warnings for pointer overflow. The index might be negative at times,
but the index will not be used to address the arrays while negative.

Bug: webrtc:9166
Change-Id: I3a32d8e814660f43be9d4c94889d00ac3f8403a5
Reviewed-on: https://webrtc-review.googlesource.com/71165
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Commit-Queue: Henrik Lundin <b9599afb29b1fa4305dba8bb04f760bf0c9c9471@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22951}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc