gas pending_bundle_size assert

oss-fuzz managed to trigger this assert, by assembling directives in
the absolute section.  Avoid this using similar code to that in
frags.c:frag_new (commit 2dc2dfa7d7a5).

gas/read.c

diff --git a/gas/read.c b/gas/read.c
index d17e171..0e04fcb 100644
--- a/gas/read.c
+++ b/gas/read.c
@@ -712,7 +712,7 @@
   if (frag->fr_type == rs_machine_dependent)
     size += md_frag_max_var (frag);
 
-  gas_assert (size >= offset);
+  gas_assert (size >= offset || now_seg == absolute_section);
 
   return size - offset;
 }