fuzzer.c: enabled ring buffer tests for decompress_fast

fuzzer.c: enabled ring buffer tests for decompress_fast

Ring buffer tests were performed only with LZ4_decompress_safe_continue,
leaving my buggy changes to LZ4_decompress_safe_continue undetected.
The tests are now replicated and performed in a similar manner for both
LZ4_decompress_safe_continue and LZ4_decompress_safe_continue (except
for the small buffer case where only one function can be tested,
because part of the dictionary is overwritten with the output).

I also updated function names in the messages (changed them to the
actual ones).  The error was reported for LZ4_decompress_safe(),
which I found misleading.