Add null checks in tensor parser to prevent SIGSEGV

[rascani]

Summary:
Add two defensive checks in tensor_parser_exec_aten.cpp:

1. getMemPlannedPtr(): Validate that the HierarchicalAllocator pointer is non-null before dereferencing it. A null allocator causes a SIGSEGV when a model has memory-planned tensors but the caller provides zero planned buffers (T266226256 — wearables Android crash at 0.0003% hit rate).

2. getTensorDataPtr(): When a tensor has allocation_info (indicating it expects memory-planned storage), validate that the allocator is non-null before any code path reaches getMemPlannedPtr(). This is the primary fix; the check in getMemPlannedPtr is defense-in-depth.

Differential Revision: D103467786

[pytorch-bot]

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❌ 2 New Failures, 4 Unrelated Failures

As of commit f0fa552 with merge base a3dd0fa ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t c98b8faf9b5b30db090b506048d05ab3bf8df7c9ae46f6a84eb48dfe1d3ab881 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-coreml-bc-macos (macos-m2-stable) / macos-job (gh) (matched macos rule in flaky-rules.json)
  File doesn't exist
• pull / test-lora-multimethod-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@rascani has exported this pull request. If you are a Meta employee, you can view the originating Diff in D103467786.

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