Update get_time_info() to use mult_frac() for precise time calculations

Refactor the get_time_info() function to use the newly added
mult_frac() function for calculations involving time conversions. This
change improves precision and mitigates the risk of overflow in the
following areas:

- Converte the computation of nanoseconds from mach_absolute_time()
  using the timebase information.
- Update the calculation of nanoseconds from clock() in low-resolution
  timer mode.

These updates make the time calculations in get_time_info() more robust
and reliable by leveraging the precise arithmetic capabilities of
mult_frac(). This change enhances the overall accuracy of time-related
computations in the code.

Author: visitorckw

src/utils.c

@@ -53,14 +53,13 @@ static void get_time_info(int32_t *tv_sec, int32_t *tv_nsec)
      */
     if (info.denom == 0)
         (void) mach_timebase_info(&info);
-    /* Hope that the multiplication doesn't overflow. */
-    uint64_t nsecs = mach_absolute_time() * info.numer / info.denom;
+    uint64_t nsecs = mult_frac(mach_absolute_time(), info.numer, info.denom);
     *tv_sec = nsecs / 1e9;
     *tv_nsec = nsecs - (*tv_sec * 1e9);
 #else /* low resolution timer */
     clock_t t = clock();
     *tv_sec = t / CLOCKS_PER_SEC;
-    *tv_nsec = (t % CLOCKS_PER_SEC) * (1e9 / CLOCKS_PER_SEC);
+    *tv_nsec = mult_frac(t % CLOCKS_PER_SEC, 1e9, CLOCKS_PER_SEC);
 #endif
 }