Factor out od_ec_dec_refill().
Factor out od_ec_dec_refill().

The same functionality was being used in two different places.
This factors it out.
The result is fully inlined by the compiler and in fact at least
 for LLVM the resulting object file is instruction-by-instruction
 identical.

Signed-off-by: Timothy B. Terriberry <87541630b4ff107b8c5639c7a8b1963cfbab1d1b@xiph.org>

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