avutil/intfloat_readwrite: avoid comparission with INFINITY, use isinf()

Should fix pgc warning
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/intfloat_readwrite.c

@@ -88,7 +88,7 @@ AVExtFloat av_dbl2ext(double d){
             ext.mantissa[i] = m>>(56-(i<<3));
     } else if (f != 0.0) {
         ext.exponent[0] = 0x7f; ext.exponent[1] = 0xff;
-        if (f != INFINITY)
+        if (!isinf(f))
             ext.mantissa[0] = ~0;
     }
     if (d < 0)