Monobit cannot create images when code point labels are missing

[hackerb9]

Several fonts do not have code point labels. Monobit can correctly create PDF files from them, but when asked to generate a PNG, it states, "ERROR: No codepoint labels found" and gives up.

While this might be a flaw in the individual font files, it would be nice if Monobit would not die on them.

Details

$ ./mkfontimages 
./teletext/saa5057-cyrillic-datasheet.yaff images/teletext/saa5057-cyrillic-datasheet.png
ERROR: No codepoint labels found.
./hellschreiber/facit-5x7.yaff images/hellschreiber/facit-5x7.png
ERROR: No codepoint labels found.
./hellschreiber/abtastfernschreiber-8x14.yaff images/hellschreiber/abtastfernschreiber-8x14.png
ERROR: No codepoint labels found.
./hellschreiber/hell80-7x9.yaff images/hellschreiber/hell80-7x9.png
ERROR: No codepoint labels found.
./hellschreiber/t-typ72-gl-7x14.yaff images/hellschreiber/t-typ72-gl-7x14.png
ERROR: No codepoint labels found.
./hellschreiber/gtl-telewriter-7x8.yaff images/hellschreiber/gtl-telewriter-7x8.png
ERROR: No codepoint labels found.
./hellschreiber/feld-hell-tbs24a-7x14-cyrillic.yaff images/hellschreiber/feld-hell-tbs24a-7x14-cyrillic.png
ERROR: No codepoint labels found.
./hellschreiber/feld-hell-tbs24a-7x14.yaff images/hellschreiber/feld-hell-tbs24a-7x14.png
ERROR: No codepoint labels found.
./next/Lexi/Lexi_14.yaff images/next/Lexi/Lexi_14.png
ERROR: No codepoint labels found.
./next/Lexi/Lexi_10.yaff images/next/Lexi/Lexi_10.png
ERROR: No codepoint labels found.
./next/Lexi/Lexi_12.yaff images/next/Lexi/Lexi_12.png
ERROR: No codepoint labels found.
./next/Lexi/Lexi_18.yaff images/next/Lexi/Lexi_18.png
ERROR: No codepoint labels found.
./next/Symbol/Symbol_18.yaff images/next/Symbol/Symbol_18.png
ERROR: No codepoint labels found.
./next/Symbol/Symbol_12.yaff images/next/Symbol/Symbol_12.png
ERROR: No codepoint labels found.
./next/Symbol/Symbol_10.yaff images/next/Symbol/Symbol_10.png
ERROR: No codepoint labels found.
./next/Symbol/Symbol_14.yaff images/next/Symbol/Symbol_14.png
ERROR: No codepoint labels found.
./banner/cedar-banner.yaff images/banner/cedar-banner.png
ERROR: No codepoint labels found.