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.
diff --git a/include/common/buffer.h b/include/common/buffer.h
index f11d6a9..c6fb2fe 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -543,7 +543,7 @@
 		return 2;
 	}
 
-	*blk1 = buf->p - buf->o;
+	*blk1 = bo_ptr(buf);
 	*len1 = buf->o;
 	return 1;
 }