checkpolicy: use YYerror only when available

The special error value YYerror is only available since bison 3.6
(released 2020).  For example the version used by oss-fuzz does not
support it.

Use a special token in case YYerror is not available.  Only downside is
a duplicate error message, one from the manual yyerror() call and one
from within bison for the unexpected special token (which would be
omitted by using YYerror).

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

checkpolicy/policy_parse.y

@@ -153,6 +153,7 @@ typedef int (* require_func_t)(int pass);
 %token FILESYSTEM
 %token DEFAULT_USER DEFAULT_ROLE DEFAULT_TYPE DEFAULT_RANGE
 %token LOW_HIGH LOW HIGH GLBLUB
+%token INVALID_CHAR
 
 %left OR
 %left XOR

checkpolicy/policy_scan.l

@@ -308,7 +308,14 @@ GLBLUB				{ return(GLBLUB); }
 "]" |
 "~" |
 "*"				{ return(yytext[0]); } 
-.                               { yyerror("unrecognized character"); return YYerror; }
+.                               { yyerror("unrecognized character");
+/* Available since bison 3.6, avoids duplicate error message */
+#ifdef YYerror
+				  return YYerror;
+#else
+				  return INVALID_CHAR;
+#endif
+				}
 %%
 int yyerror(const char *msg)
 {