MINOR: buffer: split bi_contig_data() into ci_contig_data and b_config_data()

This function was sometimes used from a channel and sometimes from a buffer.
In both cases it requires knowledge of the size of the output data (to skip
them). Here the split ensures the channel can deal with this point, and that
other places not having output data can continue to work.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 8aab234..2f41c6f 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2838,7 +2838,7 @@
 	/* Block1 is the length of the first block before the buffer wraps,
 	 * block2 is the optional second block to reach the end of the frame.
 	 */
-	block1 = bi_contig_data(h2c->dbuf);
+	block1 = b_contig_data(h2c->dbuf, 0);
 	if (block1 > flen)
 		block1 = flen;
 	block2 = flen - block1;