internal/postgres: fix panic on insert
internal/postgres: fix panic on insert

Change-Id: I5be205c916be5deed72901e92e77e60b37408037
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259099
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite