Fix off-by-one error in String.cstring

[mfelsche]

that could surface if the newly allocated cstring buffer is not set to 0 .

Fixes #2613

[slfritchie]

This patch fixes an intermittent junk byte that I see in a Wallaroo integration test that prints output with @printf[I32]("output: %s\n".cstring(), s.cstring()), where s is generated within a Wallaroo pipeline. The junk byte would appear after the last character of s and the newline.

Before the patch, the fraction of lines with a junk byte is something like 0.5% of printed list. With this patch, 0% of lines have a junk byte.