BUG/MAJOR: buffers: fix get_buffer_nc() for data at end of buffer

This function incorrectly dealt with the case where data doesn't
wrap but lies at the end of the buffer, resulting in Lukas' reported
data corruption with HTTP/2. No backport is needed, it was introduced
for HTTP/2 in 1.8-dev.

include/common/buffer.h

@@ -543,7 +543,7 @@ static inline int bo_getblk_nc(struct buffer *buf, char **blk1, int *len1, char
 		return 2;
 	}
 
-	*blk1 = buf->p - buf->o;
+	*blk1 = bo_ptr(buf);
 	*len1 = buf->o;
 	return 1;
 }