Avoid potential index-out-of-bounds

flow/Trace.cpp

@@ -1220,6 +1220,6 @@ void TraceEventFields::validateFormat() const {
 }
 
 std::string traceableStringToString(const char* value, size_t S) {
-	ASSERT_WE_THINK(value[S - 1] == '\0');
+	ASSERT_WE_THINK(S > 0 && value[S - 1] == '\0');
 	return std::string(value, S - 1); // Exclude trailing \0 byte
 }

flow/Trace.h

@@ -266,6 +266,7 @@ std::string traceableStringToString(const char* value, size_t S);
 
 template<size_t S>
 struct TraceableString<char[S]> {
+	static_assert(S > 0, "Only string literals are supported.");
 	static const char* begin(const char* value) {
 		return value;
 	}