bfd: avoid a crash when debug_section isn't created in XCOFF

bfd/
	* xcofflink.c (bfd_xcoff_size_dynamic_sections):
	Add check to know if debug_section is initialized.
diff --git a/bfd/xcofflink.c b/bfd/xcofflink.c
index e18efd4..a00ae89 100644
--- a/bfd/xcofflink.c
+++ b/bfd/xcofflink.c
@@ -3992,7 +3992,8 @@
 	}
     }
 
-  if (info->strip != strip_all)
+  if (info->strip != strip_all
+      && xcoff_hash_table (info)->debug_section != NULL)
     xcoff_hash_table (info)->debug_section->size =
       _bfd_stringtab_size (debug_strtab);