[lldb/Core] Fix crash in ValueObject::CreateChildAtIndex

The patch fixes a crash in ValueObject::CreateChildAtIndex caused by a
null pointer dereferencing. This is a corner case that is happening when
trying to dereference a variable with an incomplete type, and this same
variable doesn't have a synthetic value to get the child ValueObject.

If this happens, lldb will now return a null pointer that will results
in an error message.

rdar://65181171

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

lldb/source/Core/ValueObject.cpp

@@ -687,10 +687,15 @@ ValueObject *ValueObject::CreateChildAtIndex(size_t idx,
         language_flags);
   }
 
-  if (!valobj && synthetic_array_member)
-    valobj = GetSyntheticValue()
-                 ->GetChildAtIndex(synthetic_index, synthetic_array_member)
-                 .get();
+  // In case of an incomplete type, LLDB will try to use the ValueObject's
+  // synthetic value to create the child ValueObject.
+  if (!valobj && synthetic_array_member) {
+    if (ValueObjectSP synth_valobj_sp = GetSyntheticValue()) {
+      valobj = synth_valobj_sp
+                   ->GetChildAtIndex(synthetic_index, synthetic_array_member)
+                   .get();
+    }
+  }
 
   return valobj;
 }