Fix warnings: option.c: makeset()/put_setnum(): Various (3): FP.

Problems   : Dereference of null pointer @ 6251.
             Dereference of null pointer @ 6267.
             Dereference of null pointer @ 6351.
Diagnostic : False positive.
Rationale  : Problems occur if varp is NULL after
             `varp = get_varp_scope(p, opt_flags);`.
             That can only happen if option is hidden. Those are options
             that can be set (for backwards compatibility reasons) but
             that do nothing (see `:h hidden-options`,
             `:h missing-options`). In particular, even if setting them
             is allowed, value is not stored, so these options have no
             real value.
             So, suggested error paths should not occur, as checks
             comparing option value and default value should discard
             them.
Resolution : We could just `assert(varp)` before line 6235
             `varp_local = varp;`. That was tried and worked.
             But we prefer modifying code more readable by making
             explicit that hidden options should be skipped.
             A redundant `!istermoption(p)` is removed (it's also
             checked by for loop condition).