isc_buffer_copyregion() is broken for auto-reallocated buffers

While isc_buffer_copyregion() calls isc_buffer_reserve() to ensure the target buffer will have enough available space to append the contents of the source region to it, the variables used for subsequently checking available space are not updated accordingly after that call. This prevents isc_buffer_copyregion() from working as expected for auto-reallocated buffers: ISC_R_NOSPACE will be returned if enough space is not already available in the target buffer before it is reallocated.