Answer
The angular momentum law would not be violated.
Work Step by Step
The $\pi^-$ has spin $0$
The down quark $d$ has spin $\frac{1}{2}$
The up quark $u$ has spin $\frac{1}{2}$
If one of the quarks has spin angular momentum of $\frac{1}{2}~\hbar$, the other quark can have spin angular momentum of $-\frac{1}{2} \hbar$. Then the net spin angular momentum is zero.
The angular momentum law would not be violated.
