vrend: silence false-positive use-of-uninitialized-value reported by fuzzer

Fuzzer detected a Use-of-uninitialized-value in p_atomic_dec_zero(), which uses inline assembly on some platforms. MSAN's documentation claims unreliable results when instrumenting functions with inline assembly. In this case, unsigned char c is write-only and it's initial value isn't important, just disable MSAN for p_atomic_dec_zero().

Signed-off-by: Ryan Neph ryanneph@google.com