[Demangler] Null-check a pointer before using it in arithmetic

This addresses a UBSan -fsanitize=null diagnostic seen when compiling
the stdlib:

runtime error: applying non-zero offset 128 to null pointer

Author: vedantk

include/swift/Demangling/Demangler.h

@@ -140,7 +140,7 @@ class NodeFactory {
 #endif
 
     // Do we have enough space in the current slab?
-    if (CurPtr + ObjectSize > End) {
+    if (!CurPtr || CurPtr + ObjectSize > End) {
       // No. We have to malloc a new slab.
       // We double the slab size for each allocated slab.
       SlabSize = std::max(SlabSize * 2, ObjectSize + alignof(T));