rrdtool: Fix strncat() usage

rrdtool: Fix strncat() usage

As per strncat man page:

       If src contains n or more bytes, strncat() writes n+1 bytes to
       dest (n from src plus the terminating null byte).  Therefore,
       the size of dest must be at least strlen(dest)+n+1.

Therefore, we must ensure the destination buffere does not overflow is
src is large enough.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>