checkpolicy: return YYerror on invalid character

Inform bison about an invalid character by returning YYerror, so the
parser can cleanup internal state and return the failure via yyparse().
Currently the error is only observable via the global variable
policydb_errors, which needs to be checked separately.

Reported-by: oss-fuzz (issue #67270)
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

checkpolicy/policy_scan.l

@@ -308,7 +308,7 @@ GLBLUB				{ return(GLBLUB); }
 "]" |
 "~" |
 "*"				{ return(yytext[0]); } 
-.                               { yyerror("unrecognized character");}
+.                               { yyerror("unrecognized character"); return YYerror; }
 %%
 int yyerror(const char *msg)
 {