SDL_iconv: backport multiple fixes from SDL2:

- fixes to detection of overlong sequences,
- fixes to UCS2/UCS4 conversions,
- fixes to invalid out of bounds UTF8 handling,
- fixes SDL_iconv_string() to default to UTF-8,
- always allocate zt in output of SDL_iconv_string,
- fix compiler warnings, etc.

Backport from SDL2 commits:
53e5844bc600d265804c8e660b1dbeb89e9f1d55
4c2eb6b95d3cc3f9bba597450da00780aa7e750d
363a645ffdf742d09e9458876b617169ab08cdf3
559be8aab4a0e666fa6fc9104570c9c9d3c54f12
9772d0512c8a0bb1841244ef9043e598ba0c0ff7
945a364009f3f1febf5e165ad096418457d9baa6
a4624a59cd599dc4d9b67b187d1f275099f6f24a
8df045cc7d6ca21dd8ebbbe11ca24dfe81851f3b
df15b7d33c498aca06aa0a7211b50af6ebd4ef09
2660da6f5c845a19b898ae67dfad821baa346046
52d63ba26c2abcfcabbf5c644eaf464a104de8e1
b019d548871f879dc92de7a592154400a6efbd2c