libdwfl: Check mod is not NULL in check_module.

check_module is called from dwfl_module_relocate_address and from
dwfl_module_address_section. Both could take a NULL Dwfl_Module if
an earlier error had occured. Make check_module return immediately
indicating an issue in that case.

Signed-off-by: Mark Wielaard <mjw@redhat.com>

libdwfl/ChangeLog

@@ -1,3 +1,7 @@
+2015-05-24  Mark Wielaard  <mjw@redhat.com>
+
+	* derelocate.c (check_module): Check mod is not NULL.
+
 2015-05-22  Mark Wielaard  <mjw@redhat.com>
 
 	* link_map.c (dwfl_link_map_report): Allocate phdrs and dyn with

libdwfl/derelocate.c

@@ -285,6 +285,9 @@ dwfl_module_relocation_info (Dwfl_Module *mod, unsigned int idx,
 static bool
 check_module (Dwfl_Module *mod)
 {
+  if (mod == NULL)
+    return true;
+
   if (INTUSE(dwfl_module_getsymtab) (mod) < 0)
     {
       Dwfl_Error error = dwfl_errno ();