debuginfod-client.c: Skip empty file creation for cancelled queries

debuginfod-client.c: Skip empty file creation for cancelled queries

Empty files in the client cache are used to indicate that contacted
servers could not find a requested resource.  Future queries for this
resource will not be attempted until the cache_miss_s duration has
passed.

Currently these empty files are also created when a query is cancelled
through the client's progressfn.  This can occur, for example, when a
user cancels a download with ctrl-c.

This prevents user-cancelled queries from being retried promptly without
having to modify cache_miss_s.  Fix this by skipping the creation of an
empty cache file when progressfn cancels a query.

Signed-off-by: Aaron Merey <amerey@redhat.com>