batmand: Don't dereference orig_node before checking for NULL

The update_route functions first stores the orig_node->router in an extra
variable and later checks if orig_node is NULL. This is not only a potential
cause of a crash but can also cause new compilers to drop the NULL check
completely [1].

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fdelete-null-pointer-checks-843

Signed-off-by: Sven Eckelmann <sven@narfation.org>