Safeguard against suspiciously large allocations.

If we run into this check, likely some computation has overflowed, but
we're getting these values from many different type systems and
language plugins, so it's still good to check it at the very end, even
if these are bugs to fixed on their own.

rdar://108385563

Author: adrian-prantl

lldb/source/Plugins/Language/Swift/SwiftUnsafeTypes.cpp

@@ -511,9 +511,14 @@ bool lldb_private::formatters::swift::UnsafeTypeSyntheticFrontEnd::Update() {
     return false;
 
   m_element_stride = *stride;
-
   if (m_children.empty()) {
     size_t buffer_size = num_children * m_element_stride;
+    if (buffer_size > 512*1024*1024) {
+      LLDB_LOG(GetLog(LLDBLog::DataFormatters),
+               "Suspiciously large object: num_children={0}, stride={1}",
+               num_children, m_element_stride);
+      return false;
+    }
     m_buffer_sp.reset(new DataBufferHeap(buffer_size, 0));
 
     Status error;