[libSyntax] Don't set hot use memory region if source is empty

If the source is empty, the start of the copied buffer is a nullptr which doesn't live inside the SyntaxArena's bump allocator. Thus the assert inside `setHotUseMemoryArea` fails.

Author: ahoppen

lib/SyntaxParse/SyntaxTreeCreator.cpp

@@ -36,8 +36,10 @@ SyntaxTreeCreator::SyntaxTreeCreator(SourceManager &SM, unsigned bufferID,
   const char *Data = BufferContent.data();
   Arena->copyStringToArenaIfNecessary(Data, BufferContent.size());
   ArenaSourceBuffer = StringRef(Data, BufferContent.size());
-  Arena->setHotUseMemoryRegion(ArenaSourceBuffer.begin(),
-                               ArenaSourceBuffer.end());
+  if (!ArenaSourceBuffer.empty()) {
+    Arena->setHotUseMemoryRegion(ArenaSourceBuffer.begin(),
+                                 ArenaSourceBuffer.end());
+  }
 }
 
 SyntaxTreeCreator::~SyntaxTreeCreator() = default;