Fix integer overflow in PlatformMemoryAllocator::allocate() (TOB-EXECUTORCH-26)

Add overflow checking before computing the total allocation size
(sizeof(AllocationNode) + size + alignment) in PlatformMemoryAllocator::allocate().
Previously, when this sum exceeded SIZE_MAX, it would wrap around to a small
value, causing pal_allocate to allocate an undersized buffer. This could lead to
subsequent out-of-bounds writes. The fix validates each addition step against
SIZE_MAX and returns nullptr on overflow.

This PR was authored with the assistance of Claude.

Author: Github Executorch

runtime/executor/platform_memory_allocator.h

@@ -12,6 +12,7 @@
 #include <cinttypes>
 #include <cstdint>
 
+#include <c10/util/safe_numerics.h>
 #include <executorch/runtime/core/memory_allocator.h>
 #include <executorch/runtime/platform/log.h>
 #include <executorch/runtime/platform/platform.h>
@@ -46,8 +47,19 @@ class PlatformMemoryAllocator final : public MemoryAllocator {
       return nullptr;
     }
 
-    // Allocate enough memory for the node, the data and the alignment bump.
-    size_t alloc_size = sizeof(AllocationNode) + size + alignment;
+    // Check for overflow before computing total allocation size.
+    // Then allocate enough memory for node, data and the alignment bump.
+    size_t alloc_size = 0;
+    if (c10::add_overflows(sizeof(AllocationNode), size, &alloc_size) ||
+        c10::add_overflows(alloc_size, alignment, &alloc_size)) {
+      ET_LOG(
+          Error,
+          "Allocation size overflow: size %zu, alignment %zu",
+          size,
+          alignment);
+      return nullptr;
+    }
+
     void* node_memory = runtime::pal_allocate(alloc_size);
 
     // If allocation failed, log message and return nullptr.