Call OnLinking earlier.

Author: teo-tsirpanis

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/BlobBuilder.cs

@@ -602,6 +602,8 @@ public void LinkSuffix(BlobBuilder suffix)
                 return;
             }
 
+            OnLinking(this, suffix);
+
             bool isEmpty = Count == 0;
 
             // swap buffers of the heads:
@@ -614,8 +616,6 @@ public void LinkSuffix(BlobBuilder suffix)
             _buffer = suffixBuffer;
             _length = suffixLength;
 
-            OnLinking(this, suffix);
-
             PreviousLength += suffix.Length + suffixPreviousLength;
 
             // Update the _previousLength of the suffix so that suffix.Count = suffix._previousLength + suffix.Length doesn't change.