Fix loading PSB with cinf tag

[SRHMorris]

Description

Fixes #2846

I realise that this key isn't specified in the documentation as one with an 8 byte length, but this certainly appears to fix this issue and causes the "remaining" variable in load_global_additional() to drop to exactly zero.

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.