gwinhttpvfs: Fall back to wrapped VFS if creating a HTTP file fails

If we fail to create a GWinhttpFile for a URI (for example, because it’s
an invalid URI or is badly encoded), don’t just return NULL. Instead,
fall back to the wrapped VFS which might be able to handle it instead.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1819
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