`PlatformList::Create()` added an item to the list even when
`Platform::Create()` returned `nullptr`. Other methods use these items
without checking, which can lead to a crash. For example:
```
> lldb
(lldb) platform select unknown
error: unable to find a plug-in for the platform named "unknown"
(lldb) file a.out-arm64
PLEASE submit a bug report to...
Stack dump:
0. Program arguments: lldb
1. HandleCommand(command = "file a.out-arm64 ")
...
```
f8a4f61101