Fix segfault when there are too many closing round brackets in parsed code

Examples/test-suite/errors/cpp_extra_brackets.i

@@ -0,0 +1,6 @@
+%module cpp_extra_brackets
+
+// Extra brackets was segfaulting in SWIG-3.0.0
+struct ABC {
+  int operator<<(ABC &))) { return 0; }
+};

Examples/test-suite/errors/cpp_extra_brackets.stderr

@@ -0,0 +1 @@
+cpp_extra_brackets.i:5: Error: Syntax error in input(3).

Source/Swig/scanner.c

@@ -345,7 +345,8 @@ Scanner_push_brackets(Scanner *s) {
  * ----------------------------------------------------------------------------- */
 void
 Scanner_pop_brackets(Scanner *s) {
-  Delitem(s->brackets, 0);
+  if (Len(s->brackets) > 0) /* protect against unbalanced ')' brackets */
+    Delitem(s->brackets, 0);
 }
 
 /* -----------------------------------------------------------------------------