Image.Color may dump core on "grey"-input

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=982

Reported by Martin Nilsson, IDA nilsson@pike.ida.liu.se

Pike v7.1 release 21 running Hilfe v2.0 (Incremental Pike Frontend)
> Image.Color("grey00");
Result: "û"
> Image.Color("grey99");
/home/nilsson/Pike/7.1/src/pike_types.c:153: Fatal error:
Error in type string 32.
Attempting to dump backlog (may fail)...
Backtrace at time of fatal:
-:1: ___Foo4711()
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:103:
    do_evaluate("mixed ___Foo4711() { return
(mixed)(Image.Color(\"grey99\"));
}\n",1)
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:603:
parse_statement("Image.Color(\"grey99\");")
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:381:
do_parse()
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:224:
add_buffer("Image.Color(\"grey99\");\n")
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:623:
add_input_line("Image.Color(\"grey99\");\n")
/home/nilsson/Pike/pike/7.1.21/lib/modules/Tools.pmod/Hilfe.pmod:703:
create()
Hilfe: StdinHilfe()
Abort (core dumped)