lint: fix initialization for arrays with designators

From the previous commit, there was an off-by-one error left, which was
due to the interaction between designation_add_subscript and
extend_if_array_of_unknown_size.

The other crucial point was to call initstack_pop_nobrace before
accessing the "current initialization stack element".  Without this
call, in msg_168.c the "current element" would point to the initializer
level for 'const char *' instead of the one for 'array of const char *'.

One more step towards supporting C99.

(rillig)