examples/http2-upload: fix a crash #5463
In addition, mitigate some possible crashes of similar origin.
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.
'make checksrc' found a few code style nits: