Avoid division-by-zero when tmap search returns two tmap entries

with the same vobu_idx (which can happen at the end of a title).
4 jobs for master in 2 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #298013
amd64 debian
build-debian

00:00:40

failed #298014
macos
build-macos

00:00:20

passed #298016
win32 amd64 debian
build-win32

00:02:09

passed #298015
win64 amd64 debian
build-win64

00:02:09

 
Name Stage Failure
failed
build-macos Build
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
HTTPS-proxy has similar options --proxy-cacert and --proxy-insecure.
ERROR: Job failed: Process exited with: 1. Reason was: ()