CVE-2022-50455

In the Linux kernel, the following vulnerability has been resolved: nfs: fix possible null-ptr-deref when parsing param According to commit "vfs: parse: deal with zero length string value", kernel will set the param->string to null pointer in vfs_parse_fs_string() if fs string has zero length. Yet the problem is that, nfs_fs_context_parse_param() will dereferences the param->string, without checking whether it is a null pointer, which may trigger a null-ptr-deref bug. This patch solves it by adding sanity check on param->string in nfs_fs_context_parse_param().