afl-fuzz: Fix double free

This crash was found by running afl-fuzz. When a compile error is found,
we would sometimes free the block list twice, once in compile_file() and
once again in compile_filter(). This change sets |*arg_blocks| to NULL
after freeing it to avoid that.

Bug: None
Test: make tests
Change-Id: I048e0008acd6bf386b96d2f895953bd646af156a
2 files changed