Fix -Wzero-as-null-pointer-constant warning in jvmti_common.hpp

In test/lib/jdk/test/lib/jvmti/jvmti_common.hpp the function print_method calls fflush with an argument of 0. That argument's type is FILE*, hence triggering a -Wzero-as-null-pointer-constant warning. This accounts for about 10% of the warnings generated when building with that option, all in HotSpot jvmti tests.