[binutils-gdb] Re-fix leak in source.c (open_source_file).

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 5446094655df842abb4ababac39f34c6342e8da3 ***

Author: Philippe Waroquiers <philippe.waroquiers@skynet.be>
Branch: master
Commit: 5446094655df842abb4ababac39f34c6342e8da3

Re-fix leak in source.c (open_source_file).

Leak fixed in '8e6a5953e1d Fix 4K leak in open_source_file' has been partially
undone by '2179fbc36d23 Return scoped_fd from open_source_file'. Re-add the
transfer of current s->fullname to the unique_xmalloc_ptr fullname given to
find_and_open_source.