re PR fortran/54462 (Another "segmentation fault" after an error in COMMON statement after r190853)

2012-09-05  Tobias Burnus  <burnus@net-b.de>

        PR fortran/54462
        * symbol.c (gfc_undo_symbols): Avoid NULL pointer dereference.

From-SVN: r190989

gcc/fortran/ChangeLog

+2012-09-05  Tobias Burnus  <burnus@net-b.de>
+
+	PR fortran/54462
+	* symbol.c (gfc_undo_symbols): Avoid NULL pointer dereference.
+
 2012-09-04  Janus Weil  <janus@gcc.gnu.org>
 
 	PR fortran/54435

gcc/fortran/symbol.c

@@ -2919,10 +2919,12 @@ gfc_undo_symbols (void)
 		  gfc_symtree st, *st0;
 		  st0 = find_common_symtree (p->ns->common_root,
 					     p->common_block);
-
-		  st.name = st0->name;
-		  gfc_delete_bbt (&p->ns->common_root, &st, compare_symtree);
-		  free (st0);
+		  if (st0)
+		    {
+		      st.name = st0->name;
+		      gfc_delete_bbt (&p->ns->common_root, &st, compare_symtree);
+		      free (st0);
+		    }
 		}
 
 	      if (p->common_block->head == p)