Fix condition for assuming at EOF

Part of that condition's purpose is to ensure that we read the correct
lexer state; but it's possible now for the fstack to be non-empty
*before* the lexer state is registered, i.e. if there is an error
in the function that registers it.
This causes a NULL pointer deref.

src/asm/fstack.cpp

@@ -73,7 +73,7 @@ std::string const &FileStackNode::dump(uint32_t curLineNo) const {
 }
 
 void fstk_DumpCurrent() {
-	if (contextStack.empty()) {
+	if (lexer_AtTopLevel()) {
 		fputs("at top level", stderr);
 		return;
 	}