adpcmenc: Calculate the IMA_QT predictor without overflow

Previously, the value given to put_bits was 10 bits long for positive
predictors, even though 9 bits were to be written. The extra bit could
in some cases overwrite existing bits in the bitstream writer cache.

This fixes a failed assert in put_bits.h, when running a version
built with -DDEBUG.

The fate test result gets slightly improved, thanks to getting rid
of the overwritten bits in the bitstream writer cache.

Signed-off-by: Martin Storsjö <martin@martin.st>