avutil/rational: use frexp rather than ad-hoc log to get floating point exponent

This simplifies and cleans up the code.
Furthermore, it is much faster due to absence of the slow log computation.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavutil/rational.c

@@ -106,14 +106,14 @@ AVRational av_sub_q(AVRational b, AVRational c)
 AVRational av_d2q(double d, int max)
 {
     AVRational a;
-#define LOG2  0.69314718055994530941723212145817656807550013436025
     int exponent;
     int64_t den;
     if (isnan(d))
         return (AVRational) { 0,0 };
     if (fabs(d) > INT_MAX + 3LL)
         return (AVRational) { d < 0 ? -1 : 1, 0 };
-    exponent = FFMAX( (int)(log(fabs(d) + 1e-20)/LOG2), 0);
+    frexp(d, &exponent);
+    exponent = FFMAX(exponent-1, 0);
     den = 1LL << (61 - exponent);
     // (int64_t)rint() and llrint() do not work with gcc on ia64 and sparc64
     av_reduce(&a.num, &a.den, floor(d * den + 0.5), den, max);