tools, bpftool: Fix memory leak in codegen error cases

Free the memory allocated for the template on error paths in function
codegen.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200610130804.21423-1-tklauser@distanz.ch