Fix quad_concat when an argument is NULL

Do not try to dereference a NULL pointer while still providing a
meaningful result.

Signed-off-by: Celelibi <celelibi@gmail.com>
Little refactoring of quad.c

Breaking long lines and prefer early function exit than a big part of
code under an if.

Signed-off-by: Celelibi <celelibi@gmail.com>
Add \n at the end of MIPS instructions

Because they're needed.

Signed-off-by: Celelibi <celelibi@gmail.com>
Generate quads all the way up

And produce some MIPS code, even if it's wrong and incomplete.

Signed-off-by: Celelibi <celelibi@gmail.com>