Fixes two warnings:
libavfilter/af_afir.c:194:45: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
     int dx = FFABS(x1-x0), sx = x0 < x1 ? 1 : -1;
                                 ~~~~~~~~~~~~^~~~
libavfilter/af_aiir.c:689:45: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
     int dx = FFABS(x1-x0), sx = x0 < x1 ? 1 : -1;
                                 ~~~~~~~~~~~~^~~~