[NTG-context] Bug in startalign

[Stefan Haller]

Hi!

I (presumably) found a bug in the recent math changes of the latest
upload. If a cell of the startalign environment is empty, something
internally to the macro seems to be breaking.

Minimal example:

\starttext

\startformula
  \startalign
    \NC   \NC a \NR
    \NC b \NC c \NR
  \stopalign
\stopformula

\stoptext

Error: 
> tex error > tex error on line 5 in file ./test.tex: Missing number, treated as zero
> <to be read again>
>     \23C>1
> <template>
>     \math_next_in_eqalign \math_left_of_eqalign \mathbeginclass \csname \??mathendclass \the \numexpr \c_math_eqalign_column -\plusone \relax \endcsname
>     \startforceddisplaymath \everycr \emptytoks
> <to be read again>
>     a
> <line 3.5>

Filling the empty cell with e.g. "d" makes it work. Using "\strut" or
"\quad" or similar also leads to the same error.

Best regards,
Stefan Haller