[ptex] \mathcode 128--255 が正しく効かない

mathcode-128.tex

%#!ptex -ini
\catcode`\{=1
\catcode`\}=2
\catcode`\$=3
\catcode`\^=7

\font\FC=ec-lmtt10 at 5pt
\font\FB=ec-lmtt10 at 7pt
\font\FA=ec-lmtt10
\textfont0=\FA \scriptfont0=\FB \scriptscriptfont0=\FC

\font\IC=ec-lmtti10 at 5pt
\font\IB=ec-lmtti10 at 7pt
\font\IA=ec-lmtti10
\textfont1=\IA \scriptfont1=\IB \scriptscriptfont1=\IC

\font\SC=cmsy5
\font\SB=cmsy7
\font\SA=cmsy10 % math symbols
\textfont2=\SA \scriptfont2=\SB \scriptscriptfont2=\SC

\font\EA=cmex10 % math extension
\textfont3=\EA \scriptfont3=\EA \scriptscriptfont3=\EA

\fam0    \FA

\mathcode"79="5AB
\mathcode"80="5CD
\message{\fontname\textfont5}% => nullfont
\chardef\AB="79
\chardef\CD="80

\hsize=300pt \vbox{$[^^79][^^80]\AB\CD$}

\end

See texjporg/tex-jp-build#129