Timestamp: Prevent integer overflow on Darwin PPC 32-bit

A Darwin PPC 32-bit user observes huge values numer == 1000000000 and
denom == 18431683 returned from mach_timebase_info().  For these
values, mach_absolute_time() * numer overflows uint64_t every 1000.82
seconds, and 1000000 * denom always overflows uint32_t, with the
effect of making time run backwards at -11190660 times its usual
speed.

This bug was masked on Darwin x86 64-bit, where numer == denom == 1.

Fix it by doing the conversion with double arithmetic instead.

Closes #479.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/util/timestamp.cc

@@ -72,14 +72,16 @@ void freeze_timestamp( void )
   }
 #elif HAVE_MACH_ABSOLUTE_TIME
   static mach_timebase_info_data_t s_timebase_info;
+  static double absolute_to_millis;
 
   if (s_timebase_info.denom == 0) {
     mach_timebase_info(&s_timebase_info);
+    absolute_to_millis = 1e-6 * s_timebase_info.numer / s_timebase_info.denom;
   }
 
   // NB: mach_absolute_time() returns "absolute time units"
   // We need to apply a conversion to get milliseconds.
-  millis_cache = ((mach_absolute_time() * s_timebase_info.numer) / (1000000 * s_timebase_info.denom));
+  millis_cache = mach_absolute_time() * absolute_to_millis;
   return;								    
 #elif HAVE_GETTIMEOFDAY
   // NOTE: If time steps backwards, timeouts may be confused.