fix(memory): avoid overflow in sum of memory region lengths

The fuzzer caught an issue with the sum of the memory region lengths
that could lead to an overflow as there is no check. Replace the sum
with a try_fold to also check for overflow.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

src/vmm/src/vstate/memory.rs

@@ -297,7 +297,10 @@ pub fn snapshot_file(
     track_dirty_pages: bool,
 ) -> Result<Vec<GuestRegionMmap>, MemoryError> {
     let regions: Vec<_> = regions.collect();
-    let memory_size: u64 = regions.iter().map(|(_, size)| *size as u64).sum();
+    let memory_size = regions
+        .iter()
+        .try_fold(0u64, |acc, (_, size)| acc.checked_add(*size as u64))
+        .ok_or(MemoryError::OffsetTooLarge)?;
     let file_size = file.metadata().map_err(MemoryError::FileMetadata)?.len();
 
     // ensure we do not mmap beyond EOF. The kernel would allow that but a SIGBUS is triggered