[PROF-11524] Minor tweak: Set profile start_time after resetting (#987)

**What does this PR do?**

This PR applies a suggestion from
#963 (comment)
to set the `start_time` after obtaining the old profile.

This will hopefully make it more clear that the `start_time` is used
for the old profile, not the new one.

**Motivation:**

Make code more readable.

**Additional Notes:**

N/A

**How to test the change?**

Existing test coverage should be enough to validate this change
(for instance, the Ruby profiler test suite explicitly tests the
timestamps on profiles).

Author: ivoanjo

datadog-profiling-ffi/src/profiles/datatypes.rs

@@ -747,12 +747,11 @@ pub unsafe extern "C" fn ddog_prof_Profile_serialize(
     (|| {
         let profile = profile_ptr_to_inner(profile)?;
 
+        let mut old_profile = profile.reset_and_return_previous()?;
         if let Some(start_time) = start_time {
-            profile.set_start_time(start_time.into())?;
+            old_profile.set_start_time(start_time.into())?;
         }
 
-        let old_profile = profile.reset_and_return_previous()?;
-
         let end_time = end_time.map(SystemTime::from);
         old_profile.serialize_into_compressed_pprof(end_time, None)
     })()