Fix segfault in template simplifier

When something parsing the template syntax
went wrong, "tok2" was NULL and resulted
in deleting all remaining tokens. Whoops.

Triggered by gcc test suite:
gcc/testsuite/g++.dg/cpp0x/variadic87.C

Tracked down the source of the bug with
valgrind's "--track-origins=yes" switch.

lib/templatesimplifier.cpp

@@ -658,6 +658,11 @@ void TemplateSimplifier::useDefaultArgumentValues(const std::list<Token *> &temp
                 if (indentlevel < 0)
                     break;
             }
+            // something went wrong, don't call eraseTokens()
+            // with a nullptr "end" parameter (=all remaining tokens).
+            if (!tok2)
+                continue;
+
             Token::eraseTokens(eqtok, tok2);
             eqtok->deleteThis();
         }