patch 9.1.1041: Vim9: out-of-bound access when echoing an enum

patch 9.1.1041: Vim9: out-of-bound access when echoing an enum

Problem:  Vim9: out-of-bound access when echoing an enum
Solution: Add NUL to growarray, check that ufunc is non-null
          before accessing it to make Coverity happy
          (Yegappan Lakshmanan)

closes: #16488

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>