bpf: Initialize used but uninit variable in propagate_liveness()

With input changed == NULL, a local variable is used for "changed".
Initialize tmp properly, so that it can be used in the following:
   *changed |= err > 0;

Otherwise, UBSAN will complain:

UBSAN: invalid-load in kernel/bpf/verifier.c:18924:4
load of value <some random value> is not a valid value for type '_Bool'

Fixes: dfb2d4c ("bpf: set 'changed' status if propagate_liveness() did any updates")
Signed-off-by: Song Liu <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: liu-song-6

kernel/bpf/verifier.c

@@ -18900,7 +18900,7 @@ static int propagate_liveness(struct bpf_verifier_env *env,
 	struct bpf_reg_state *state_reg, *parent_reg;
 	struct bpf_func_state *state, *parent;
 	int i, frame, err = 0;
-	bool tmp;
+	bool tmp = false;
 
 	changed = changed ?: &tmp;
 	if (vparent->curframe != vstate->curframe) {