examples/http2-down/upload: add error checks

If `index.html` does not exist in the directory from which the example
is invoked, the fopen(upload, "rb") invocation in `setup` would fail,
returning NULL.  This value is subsequently passed as the FILE* argument
of the `fread` invocation in the `read_callback` function, which is the
actual cause of the crash (apparently `fread` assumes that argument to
be non-null).

In addition, mitigate some possible crashes of similar origin.

Closes #5463

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