profiling: don't error with statprof when profiling has already started

statprof.reset() asserts if profiling has already started. So don't
call if it profiling is already running.

mercurial/profiling.py

@@ -88,7 +88,9 b' def statprofile(ui, fp):'
 
     freq = ui.configint('profiling', 'freq', default=1000)
     if freq > 0:
-        statprof.reset(freq)
+        # Cannot reset when profiler is already active. So silently no-op.
+        if statprof.state.profile_level == 0:
+            statprof.reset(freq)
     else:
         ui.warn(_("invalid sampling frequency '%s' - ignoring\n") % freq)