_CFXMLInterface: account for possible nullptr return

`xmlSplitQName2` may return `nullptr` for the result, which when passed
to `CFStringCreateWithCString` would attempt to perform
`strlen(nullptr)` which is ill-defined. When updating libxml2 on
Windows, we would perform an invalid memory access due to the `strlen`
invocation inside `CFStringCreateWithCString`. Protect against this
case, returning `NULL` instead.

Sources/_CFXMLInterface/CFXMLInterface.c

@@ -1073,7 +1073,10 @@ CFStringRef _CFXMLNodeCopyPrefix(_CFXMLNodePtr node) {
     xmlChar* result = NULL;
     xmlChar* unused = xmlSplitQName2(_getQName((xmlNodePtr)node), &result);
 
-    CFStringRef resultString = __CFSwiftXMLParserBridgeCF.CFStringCreateWithCString(NULL, (const char*)result, kCFStringEncodingUTF8);
+    CFStringRef resultString = NULL;
+    if (result) {
+      resultString = __CFSwiftXMLParserBridgeCF.CFStringCreateWithCString(NULL, (const char*)result, kCFStringEncodingUTF8);
+    }
     xmlFree(result);
     xmlFree(unused);