Add null checks for PMU allocation in TBeamBoard

[cj-vana]

Summary

Add null safety checks after new operations for PMU initialization in TBeamBoard.cpp.

Changes

• Check if new XPowersAXP2101() succeeded before calling init()
• Check if new XPowersAXP192() succeeded before calling init()

Why

In embedded systems with limited RAM, new can fail and return nullptr. Dereferencing nullptr causes a crash. This adds defensive null checks to prevent potential crashes.

// Before
if (!PMU->init()) {

// After  
if (!PMU || !PMU->init()) {