Added an error message, in case there's a typo in the definitions of long command-line options.

src/cc65/main.c

@@ -302,6 +302,10 @@ static void FileNameOption (const char* Opt, const char* Arg, StrBuf* Name)
     if (SB_NotEmpty (Name)) {
         AbEnd ("Cannot use option `%s' twice", Opt);
     }
+    /* A typo in OptTab[] might allow a NULL Arg */
+    if (Arg == 0) {
+        Internal ("Typo in OptTab[]; option '%s' should require an argument", Opt);
+    }
     /* Remember the file name for later */
     SB_CopyStr (Name, Arg);
     SB_Terminate (Name);