tests: Fix misplaced parenthesis in qemumonitorjsontest
    
    When trying clang, it found out that we were comparing sizeof with 0
    even though we wanted to check the return value of memcmp.  That showed
    us that the test was wrong and it needs a fix as well.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>