badger: fix index-out-of-range panic on tags shorter than 2 elements

[mauromoro]

Summary

• A tag with fewer than 2 elements (e.g. NIP-70's bare ["-"] protected-event marker) has no tag[1]. The len(tag) < 2 check correctly short-circuits the rest of its compound OR condition, but the branch it falls into then unconditionally evaluates tag[1] as a call argument to IndexLongerTag — Go evaluates call arguments before the call, so this panics with an out-of-range index before IndexLongerTag's own body (which does handle a short/empty value safely) ever runs.
• Not hypothetical: once IndexLongerTag is configured, it's wired for every event a relay stores, not just ones it authors — so any client publishing any event with a bare single-element tag crashes the whole process.
• Fixed by checking len(tag) < 2 on its own, before ever touching tag[1], rather than folded into the compound condition that still (correctly) short-circuits on it but doesn't stop the later unconditional call.

Test plan

• Reproduced the panic directly (a relay configured with IndexLongerTag, an event containing a bare single-element tag) before applying the fix, confirmed it no longer panics after.
• go build, go vet, gofmt -l all clean.