ossfuzz: Improvements to json_load_dump_fuzzer
ossfuzz: Improvements to json_load_dump_fuzzer

Add in diagnostics to json_load_dump_fuzzer so that if FUZZ_VERBOSE is
defined in the environment, additional debug information about the
current test case is output to stderr.

Replace json_dumps by json_dump_callback so that memory is not exhausted
by excessively nested json objects.

bstarynk/jansson,Mephistophiles/jansson,markalanj/jansson,akheron/jansson,markalanj/jansson,Mephistophiles/jansson,akheron/jansson,akheron/jansson,markalanj/jansson,bstarynk/jansson,bstarynk/jansson