handle negative hex/octal character codes

[bcwood]

Here's an example from the included banner.flf font file:

-0x0004  KATAMAP
                                                    @
a-A i-B u-C e-D o-E ka-F ki-G ku-H ke-I ko-J        @
sa-K shi-L su-M se-N so-O ta-P chi-Q tsu-R te-S to-T@
na-U ni-V nu-W ne-X no-Y ha-Z hi-a fu-b he-c ho-d   @
ma-e mi-f mu-g me-h mo-i ya-j yu-k we-l yo-m        @
ra-n ri-o ru-p re-q ro-r wa-s wi-t wo-u             @
n-v                                                 @
                                                    @@

It's unclear to me from the FIGfont documentation how these are even meant to be handled.