It took me a day and a half, on and off, to find this

[ewx]

--- imagemagick-5.5.7.9.old/magick/blob.c       2003-05-23 19:26:28.000000000 +0100
+++ imagemagick-5.5.7.9/magick/blob.c   2003-12-31 19:25:41.000000000 +0000
@@ -418,7 +418,10 @@
   image->blob->eof=False;
   image->blob->status=status < 0;
   if (image->blob->exempt)
+  {
+    DetachBlob(image->blob);
     return;
+  }
   switch (image->blob->type)
   {
     case UndefinedStream:

...in my defence, I thought the bug was in a different place entirely for much of it.