Fix bustage from c152d602.

silk_setup_resamples() was using x_bufFIX for two purposes, and I
 only allocated enough space for one of them.
This patch also switches to slightly more descriptive variable
 names than nSamples_temp and computes the resampler input/ouput
 sizes in a way that a little more obviously doesn't have issues
 with fractional samples (and replaces a divide with a variable
 divisor by one with a constant divisor).