applypatch: Don't call inflate() when it expects zero-length output.

We may have expanded_len == 0 when calling inflate(). After switching to
using std::vector, it passes a nullptr buffer to inflate() and leads to
Z_STREAM_ERROR.

Bug: 29312140
Change-Id: Iab7c6c07a9e8488e844e7cdda76d02bd60d2ea98