Don't accept strings too long to fit in char[N] with trailing NUL.

It was previously possible to assign "hello" to a variable backed by char[5]
storage in C, and the array contained the correct character data but without
the trailing NUL, which was unexpected in C.

This is not allowed any more, only "helo" can fit into a char[5] now and
anything else fails the type check, just as it already happened for the longer
strings before.