uclient-fetch: fix overloading of output_file variable

uclient-fetch: fix overloading of output_file variable

When uclient-fetch is called with multiple URL's, it derives the
first filename from the URL. It then sets the global output_file
variable, causing a use-after-free bug on an attempt to use it as output
file for the next file.

Fix this by avoiding the overwrite entirely by only setting a local
variable

Reported-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>