perf_timer: rewrite to make it clear there is no division by zero

It could have happened if epee::misc_utils::get_ns_count
is buggy, at a push

Coverity 182561

src/common/perf_timer.cpp

@@ -49,16 +49,15 @@ namespace
 #ifdef __x86_64__
   uint64_t get_ticks_per_ns()
   {
-    uint64_t t0 = epee::misc_utils::get_ns_count();
+    uint64_t t0 = epee::misc_utils::get_ns_count(), t1;
     uint64_t r0 = get_tick_count();
 
     while (1)
     {
-      uint64_t t = epee::misc_utils::get_ns_count();
-      if (t - t0 > 1*1000000000) break; // work one second
+      t1 = epee::misc_utils::get_ns_count();
+      if (t1 - t0 > 1*1000000000) break; // work one second
     }
 
-    uint64_t t1 = epee::misc_utils::get_ns_count();
     uint64_t r1 = get_tick_count();
     uint64_t tpns256 = 256 * (r1 - r0) / (t1 - t0);
     return tpns256 ? tpns256 : 1;