trusty: Allow fuzzing without coverage

Bug: 171750250
Test: m
Change-Id: I9e4cbf11fd223092f9ad800b35d0502c27f71e8f
diff --git a/trusty/fuzz/counters.cpp b/trusty/fuzz/counters.cpp
index 1e863ac..4d34059 100644
--- a/trusty/fuzz/counters.cpp
+++ b/trusty/fuzz/counters.cpp
@@ -41,6 +41,10 @@
 namespace fuzz {
 
 ExtraCounters::ExtraCounters(coverage::CoverageRecord* record) : record_(record) {
+    if (!record_->IsOpen()) {
+        return;
+    }
+
     assert(fuzzer::ExtraCountersBegin());
     assert(fuzzer::ExtraCountersEnd());
 
@@ -51,10 +55,18 @@
 }
 
 ExtraCounters::~ExtraCounters() {
+    if (!record_->IsOpen()) {
+        return;
+    }
+
     Flush();
 }
 
 void ExtraCounters::Reset() {
+    if (!record_->IsOpen()) {
+        return;
+    }
+
     record_->ResetCounts();
     fuzzer::ClearExtraCounters();
 }