avutil/libm: fix isnan compatibility hack

Commit 14ea4151d7c3c26500193f11ac661ed20c7c2b9c had a bug in that the
conversion of the uint64_t result to an int (the return signature) would
lead to implementation defined behavior, and in this case simply
returned 0 for NAN. A fix via AND'ing the result with 1 does the trick,
simply by ensuring a 0 or 1 return value.

Patch tested with FATE on x86-64, GNU/Linux by forcing the compatibility
code via an ifdef hack suggested by Michael.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
