BUG/MINOR: buffers: Fix bi/bo_contig_space to handle full buffers

BUG/MINOR: buffers: Fix bi/bo_contig_space to handle full buffers

These functions was added in commit 637f8f2c ("BUG/MEDIUM: buffers: Fix how
input/output data are injected into buffers").

This patch fixes hidden bugs. When a buffer is full (buf->i + buf->o ==
buf->size), instead of returning 0, these functions can return buf->size. Today,
this never happens because callers already check if the buffer is full before
calling bi/bo_contig_space. But to avoid possible bugs if calling conditions
changed, we slightly refactored these functions.