Changed nimprof.hook() to handle uninitialized t0

It's unlikely, but possible for the conversion to nanoseconds
to overflow if QueryPerformanceCounter() returns a
large enough timestamp.  This change avoids that, at the
cost of always taking a sample the first time through
when t0 == 0.

lib/pure/nimprof.nim

@@ -132,7 +132,7 @@ else:
   proc hook(st: TStackTrace) {.nimcall.} =
     if interval == 0:
       hookAux(st, 1)
-    elif getTicks() - t0 > interval:
+    elif int64(t0) == 0 or getTicks() - t0 > interval:
       hookAux(st, 1)
       t0 = getTicks()