MINOR: chunks: ensure that chunk_strcpy() adds a trailing zero

Since thus function bears the name of a well-known string function, it
must at least promise compatible semantics. Here it means always adding
the trailing zero so that anyone willing to use chunk->str as a regular
string can do it. Of course the zero is not counted in the chunk's length.
(cherry picked from commit 0b6044fa241386d21994e496bc9e29851b5863dc)

[wt: not strictly needed but backported to ensure that any future patch
 relying on it works as expected]
(cherry picked from commit 88cb10e3f13c3278e3d1fb84c13dd5171ebe0d2c)
1 file changed