Fixed potential crasher in VersionedDocument

_docType wasn't being allocated on the heap, instead it just pointed
inside the doc's metadata. This caused it to be invalidated if the
metadata block got moved in memory when resized.

I haven't seen an actual crash from this, but the Xcode address sanitizer
caught it while purging docs in a unit test.

CBForest/VersionedDocument.cc

@@ -59,8 +59,10 @@ namespace cbforest {
             _unknown = true;        // i.e. doc was read as meta-only
 
         if (_doc.exists()) {
-            if (!readMeta(_doc, _flags, _revID, _docType))
+            slice docType;
+            if (!readMeta(_doc, _flags, _revID, docType))
                 throw error(error::CorruptRevisionData);
+            _docType = docType; // allocate buf for it
         } else {
             _flags = 0;
         }