[C++] small test fix (aeron-io#980)

Author: BenBudr

sbe-tool/src/test/cpp/FieldAccessOrderCheckTest.cpp

@@ -3624,7 +3624,7 @@ TEST_F(FieldAccessOrderCheckTest, allowsEncodingAndDecodingAsciiInsideGroupInSch
  AsciiInsideGroup::sbeSchemaVersion(),
  BUFFER_LEN
  );
- char aOut[6];
+ char aOut[7] = {'\0'};
  decoder.getA(aOut, 6);
  EXPECT_STREQ(aOut, "GBPUSD");
  AsciiInsideGroup::B &b = decoder.b();