encode: fix read of uninitialized target_bitrate value
encode: fix read of uninitialized target_bitrate value

Regression from commit 5eb3f15cf "Add missing file rate.c"

ycho/daala,xiph/daala,ycho/daala,xiph/daala,xiph/daala,xiph/daala,ycho/daala,ycho/daala,ycho/daala,xiph/daala