dav: Use g_try_realloc for output streams

When writing a large amount to a GMemoryOutputStream, the reallocation
function can fail.  Use g_try_realloc so that failures generate
G_IO_ERROR_NO_SPACE rather than abort the process (as happens with
g_realloc).

https://bugzilla.gnome.org/show_bug.cgi?id=739428