file: Fix uci -m import command

file: Fix uci -m import command

Without this change we see the following error:
  # uci -m import optic < /etc/optic-db/default
  uci: Parse error (option/list command found before the first section) at line 4, byte 1

ptr.last is still a null pointer in case the uci_lookup_list() call
found a matching section and set ptr.s to it. The code expects that
uci_set() updates the ptr.last pointer, but this is not done any more.
If case uci_lookup_list() did not found a section ptr->s is a null
pointer and then uci_set() will allocate a new section.

Fixes: ae61e1cad4a1 ("uci: optimize update section in uci_set")
Co-authored-by: Hauke Mehrtens <hmehrtens@maxlinear.com>
Signed-off-by: Jan Venekamp <jan@venekamp.net>