[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>