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