libbpf: Fix error handling bug in btf_dump__new

Fix missing jump to error handling in btf_dump__new, found by Coverity static
code analysis.

Fixes: 9f81654e ("libbpf: Expose BTF-to-C type declaration emitting API")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200117060801.1311525-2-andriin@fb.com