examples/sslbackend: fix -Wchar-subscripts warning

With the `isdigit` implementation that comes with MSYS2, the argument
is used as an array subscript, resulting in a -Wchar-subscripts
warning. `isdigit`'s behavior is undefined if the argument is negative
and not EOF [0]. As done in lib/curl_ctype.h, cast the `char` variable
to `unsigned char` to avoid that.

[0] https://en.cppreference.com/w/c/string/byte/isdigit

Closes https://github.com/curl/curl/pull/4503

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