zlib: don't assert on missing dictionary

Handle Z_NEED_DICT errors graciously. Fixes the following assertion:

  node: ../src/node_zlib.cc:161: static void node::ZCtx::Process
  (uv_work_t*): Assertion`ctx->dictionary_ != __null && "Stream
  has no dictionary"' failed.
  Aborted (core dumped)

Fixes #4632.
zlib: don't assert on malformed dictionary

Handle Z_DATA_ERROR errors from inflateSetDictionary() gracefully.
Fixes the following assertion:

  node: ../src/node_zlib.cc:167: static void node::ZCtx::Process
  (uv_work_t*): Assertion `ctx->err_ == 0 && "Failed to set
  dictionary"' failed.
  Aborted (core dumped)

Fixes #4632.