Bail out on NULL destfile in curl_download_internal()
Avoid a potential segfault that may occur if we use a temporary file and
fail to build the destination file name from the effective URL.
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
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Another approach is to use a random default destfile name. Since the URL
parsing is very unlikely to fail, I took the easy option here.