Answer
The energy conservation law would not be violated.
Work Step by Step
The $\pi^-$ has energy $139.6~MeV$
The down quark $d$ has energy $10~MeV$
The up quark $u$ has energy $5~MeV$
Since the energy of the $\pi^-$ is greater than the sum of the energies of the two quarks, energy can be conserved if the remaining energy is in the form of kinetic energy for the two quarks.
The energy conservation law would not be violated.
