Improve the change to allow parsing 9%a

If there was a later syntax error, this could incorrectly get
reported as:

Error: Unknown directive '%a'.

Examples/test-suite/errors/pp_unknowndirective5.i

@@ -0,0 +1,20 @@
+%module xxx
+
+/* Before SWIG 4.1.0 the `9%a` expression triggered:
+ *
+ * Error: Unknown directive '%a'
+ *
+ * The fix for that works by handling an apparent directive that isn't a
+ * recognised directive by noting down its name, emitting MODULO and then
+ * rescanning what follows, and if the parser then gives a syntax error we
+ * report it as an unknown directive.
+ *
+ * However the initial version of this failed to reset the noted down apparent
+ * directive often enough, so a later syntax error could get incorrectly
+ * reported.  Here the syntax error in the declaration of `c` was confusingly
+ * reported as `Error: Unknown directive '%a'`.  This was found and fixed prior
+ * to 4.1.0.
+ */
+int a;
+int test2(int b = 9%a) { return b; }
+void int c;

Examples/test-suite/errors/pp_unknowndirective5.stderr

@@ -0,0 +1 @@
+pp_unknowndirective5.i:18: Error: Syntax error in input(1).

Source/CParse/cscanner.c

@@ -573,6 +573,9 @@ int yylex(void) {
     scanner_init();
   }
 
+  Delete(cparse_unknown_directive);
+  cparse_unknown_directive = NULL;
+
   if (next_token) {
     l = next_token;
     next_token = 0;
@@ -951,9 +954,6 @@ int yylex(void) {
 	return (yylex());
 
     } else {
-      Delete(cparse_unknown_directive);
-      cparse_unknown_directive = NULL;
-
       /* SWIG directives */
       if (strcmp(yytext, "%module") == 0)
 	return (MODULE);