Fix memory leak in dnf_context_setup()
Fix memory leak in dnf_context_setup()

When dnf_context_setup() is called twice, there is memory leak of
original content of native_arches.
Example where dnf_context_setup() is called twice from libdnf unit tests:

ctx = dnf_context_new();
ret = dnf_context_setup(ctx, NULL, &error);

and later on:

ret = dnf_context_clean_cache(ctx, flags, &error);

The dnf_context_clean_cache() function calls dnf_context_setup() and
triggers the memory leak.

Closes: #944
Approved by: lukash

rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf