Patch https://aomedia-review.googlesource.com/c/12602/ made the
variable 'sum' in the warp filter unsigned, to indicate that its
value should always be >= 0. But 'sum' is used to accumulate
signed values, and it is expected that some of those values
will be negative.

The issue is that, when running 'x += y', if x is a uint32_t
and y is an int (and is 32 bits), the C standard says to
convert y to a uint32_t before doing the addition. This causes
overflow, and so undefined behaviour, if y < 0.

This is fixed by making 'sum' signed, and by explicitly bounds
checking against zero at the end of the filter.

BUG=aomedia:572

Change-Id: I1d484b5f5698db0ec9761807610b3b2b35647983