Fix warnings: eval.c: clear_tv(): Bad free: RI.

Problem    : Bad free @ 16076.
Diagnostic : Real issue.
Rationale  : A non-allocated string is set at 4127, which later on can
             be tried to be freed if aborting.
Resolution : Detect particular case (func with empty name) and don't
             free in that case.
             Another solution (use allocated string) was tried before,
             but it produced a leak difficult to solve.
             Finally applied solution works, but it produces a new false
             positive warning (Np dereference at 13763), deactivated by
             `assert(ptrs[i].item->li_next)`.
Fix warnings: eval.c: get_user_func_name(): Np dereference: FP.

Problem    : Dereference of null pointer @ 18216.
Diagnostic : False positive.
Rationale  : `hi` and `done` are static. Intended usage is for the first
             call to have idx == 0, so that they are initialized.
Resolution : Assert hi after (optional) initialization.
Fix warnings: eval.c: call_user_func(): Out of bounds: FP.

Problem    : Out-of-bound array access @ 18429.
Diagnostic : False positive.
Rationale  : Situation is intentional. `dictitem_T` is a prefix all dict
             items whill share, but actual size of each item will be
             different depending on its key length. `di_key` array field
             is declared of size 1 just to have a field name, but real
             size will vary for each item.
Resolution : Make analyzer ignore it.
             This could be refactored to use C99-allowed variable length
             arrays, but eval.c is bound to dissappear, so no effort is
             done in that sense.
Fix warnings: eval.c: add_nr_var(): Out of bounds: FP.

Problem    : Out-of-bound array access @ 18737.
Diagnostic : False positive.
Rationale  : Situation is intentional. `dictitem_T` is a prefix all dict
             items whill share, but actual size of each item will be
             different depending on its key length. `di_key` array field
             is declared of size 1 just to have a field name, but real
             size will vary for each item.
Resolution : Make analyzer ignore it.
             This could be refactored to use C99-allowed variable length
             arrays, but eval.c is bound to dissappear, so no effort is
             done in that sense.
Fix warnings: eval.c: do_return(): Np dereference: FP.

Problem    : Dereference of null pointer @ 18841.
Diagnostic : False positive.
Rationale  : Suggested error path takes `reanimate` branch at 18827,
             assigning `rettv = current_funccal->rettv`. Then,
             inmediately after, it supposes rettv is null, which cannot
             happen, since current_funccal->rettv should always be non
             null.
Resolution : Assert current_funccal->rettv non null.
Fix warnings: screen.c: screenalloc(): Np arg (2): MI.

Problems   : Null pointer argument in call to memory copy function
             @ 6465.
             Null pointer argument in call to memory copy function
             @ 6475.
Diagnostic : Multithreading issues.
Rationale  : Problem occurs if globals `enc_utf8` and `enc_dbcs` are
             modified while function is executing.
Resolution : Use local copy of globals.