BUG/MINOR: buffer: fix debugging condition in b_peek_varint()

The BUG_ON_HOT() test condition added to b_peek_varint() by commit
8873b85bd ("DEBUG: buf: add BUG_ON_HOT() to most buffer management
functions") was wrong as <data> in this function is not b->data,
so that was triggering during live dumps of H2 traces on the CLI
when built with -DDEBUG_STRICT=2. No backport is needed.
diff --git a/include/haproxy/buf.h b/include/haproxy/buf.h
index 3f45ee8..4ea4b73 100644
--- a/include/haproxy/buf.h
+++ b/include/haproxy/buf.h
@@ -913,7 +913,7 @@
 	uint64_t v = 0;
 	int bits = 0;
 
-	BUG_ON_HOT(ofs > data);
+	BUG_ON_HOT(ofs > b_data(b));
 
 	if (data != 0 && (*head >= 0xF0)) {
 		v = *head;