patch 9.2.0567: dict function name allocation failure not handled

patch 9.2.0567: dict function name allocation failure not handled

Problem:  When defining a dictionary function, the function name string
          is allocated with vim_strnsave() but the result is not
          checked. On allocation failure the dict entry is left with
          type VAR_FUNC and a NULL name, and in the overwrite case the
          previous entry has already been freed before the NULL is
          stored.
Solution: Allocate the name before modifying the dict entry and bail out
          on failure, freeing it on all error paths (thinca)

closes: #20376

Co-Authored-by: Claude <noreply@anthropic.com>
Signed-off-by: thinca <thinca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>