avutil/libm: fix isnan compatibility hack

Commit 14ea4151d7 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>

libavutil/libm.h

@@ -123,7 +123,7 @@ static av_always_inline av_const int avpriv_isnan(double x)
     uint64_t v = av_double2int(x);
     if ((v & 0x7ff0000000000000) != 0x7ff0000000000000)
         return 0;
-    return v & 0x000fffffffffffff;
+    return (v & 0x000fffffffffffff) && 1;
 }
 
 #define isnan(x)                  \