[RFC] coverity/68610: Fix out of bounds access

If buf is reallocated on 1708, it will be one character too short for
the terminating NULL character on 1712.
There should be no need to add one byte (one could even subtract one byte).
The format string `cntxformat` is ` <<%s>>`, so the string `context` will replace 
the (*two*) charactes `%s` in the following call to `sprintf`.