[PRISM] Fix memory leak in case nodes

The temporary array conditions_labels is heap allocated and never freed.
We can use alloca instead to stack allocate it so that we don't need to
manually free it.

For example:

    code = "case; #{100.times.map { "when #{it}; " }.join}; end"

    10.times do
      10_000.times do
        RubyVM::InstructionSequence.compile_prism(code)
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    21376
    30304
    38800
    47184
    55456
    64192
    72288
    80400
    89040
    97104

After:

    13088
    13632
    13760
    14016
    14688
    14992
    15120
    15232
    15744
    15744

0 files changed, 0 insertions, 0 deletions