Checks to avoid null dereferences in NetworkServer

Various commands in NetworkServer require extra data. However, if the
packet size is set to 0 for these, the code will skip over reading the
data in and allocating memory. This results in null dereferences.

This patch adds checks to the relevant commands to make sure they don't
continue reading a null pointer.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f