Fix Platform::New to return nullptr on failure

[kpschoedel]

Problem

Platform::New() uses placement new into a possibly-null pointer.

Summary of Changes

Check the return value of MemoryAlloc before using it.

Fixes #4998 platform::New will crash or do other bad things on allocation failure

[github-actions]

Size increase report for "gn_qpg6100-example-build" from 2891d6b

File	Section	File	VM
chip-qpg6100-lock-example.out	.text	32	32

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-qpg6100-lock-example.out and ./pull_artifact/chip-qpg6100-lock-example.out:

sections,vmsize,filesize
.debug_loc,0,473
.debug_info,0,392
.debug_line,0,48
.debug_abbrev,0,41
.text,32,32
.debug_str,0,-2
[Unmapped],0,-32
.debug_ranges,0,-96