fix buffer overflow when handling long composed input

fix buffer overflow when handling long composed input

To reproduce the issue:

"
If you already have the multi-key enabled on your system, then add this line
to your ~/.XCompose file:

[...]
<question> <T> <E> <S> <T> <question> :
"1234567890123456789012345678901234567890123456789012345678901234567890"
"

Reported by and an initial patch by Andy Gozas <andy@gozas.me>, thanks!

Adapted the patch, for now st (like dmenu) handles a fixed amount of composed
characters, or otherwise ignores it. This is done for simplicity sake.