Free temporary memory when reading TOC

Free temporary memory when reading TOC

ReadStr returns allocated memory which the caller is responsible for
freeing when done with the string. This commit ensures that memory is
freed in one case which used ReadStr in a conditional. While the leak
might not be too concerning, this makes the code consistent across all
ReadStr callsites in ReadToc. Due to the lack of complaints of issues
in production from this, no backpatch is performed at this point.

Author: Bharath Rupireddy, Georgios Kokolatos
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/oZwKiUxFsVaetG2xOJp7Hwao8F1AKIdfFDQLNJrnwoaxmjyB-45r_aYmhgXHKLcMI3GT24m9L6HafSi2ns7WFxXe0mw2_tIJpD-Z3vb_eyI=@pm.me