Make is_blob_record tolerant of invalid pickles.

src/ZODB/blob.py

@@ -745,4 +745,9 @@ def is_blob_record(record):
     storage to another.
     
     """
-    return cPickle.loads(record) is ZODB.blob.Blob
+    try:
+        return cPickle.loads(record) is ZODB.blob.Blob
+    except (MemoryError, KeyboardInterrupt, SystemExit):
+        raise
+    except Exception:
+        return False

src/ZODB/tests/testblob.py

@@ -589,7 +589,7 @@ def loadblob_tmpstore():
 """
 
 def is_blob_record():
-    """
+    r"""
     >>> fs = FileStorage('Data.fs')
     >>> bs = ZODB.blob.BlobStorage('blobs', fs)
     >>> db = DB(bs)
@@ -601,6 +601,15 @@ def is_blob_record():
     >>> ZODB.blob.is_blob_record(fs.load(ZODB.utils.p64(1), '')[0])
     True
 
+    An invalid pickle yields a false value:
+
+    >>> ZODB.blob.is_blob_record("Hello world!")
+    False
+    >>> ZODB.blob.is_blob_record('c__main__\nC\nq\x01.')
+    False
+    >>> ZODB.blob.is_blob_record('cWaaaa\nC\nq\x01.')
+    False
+
     >>> db.close()
     """