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
You'll find a mix of various documentation in this directory and
subdirectories, using several different formats. Some of them are not ideal
for reading directly in your browser.
If you'd rather see the rendered version of the documentation, check out the
curl web site's documentation section for
general curl stuff or the libcurl section for
libcurl related documentation.