_normal completion breaks when paths have colons.

Reproduction:

Call 'zsh -f'.

1. mkdir ~/foo:{a,b}
2. Type "~/foo:"<Tab>.
3. autoload -Uz compinit; compinit
4. Type "~/foo:"<Tab>

Expected outcome: Receive "~/foo:a" and "~/foo:b" as completions.

Actual outcome: The buffer is reset to simply "~".
(https://asciinema.org/a/ExvdEjMJMul6NJ2Y3rPobFiri)

I'm not subbed to the workers list, so I would appreciate an CC.
Thank you.