patch 9.1.1554: crash when omni-completion opens command-line window

patch 9.1.1554: crash when omni-completion opens command-line window

Problem:  Vim crashes during omnifunc completion inside the command-line
          window ("q:") if the completion item attempts to open an "info"
          preview window. This leads to a failed assert during execution.
Solution: Avoid opening preview windows while inside the command-line
          window to prevent the crash (Girish Palya).

closes: #17764

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>