Fix a problem I saw when some tests here failed: there's a pattern in

the code

    try:
        root = self._getRoot()
        ...
    except:
        self._closeDB(root)
	self._delDB()
	raise

which fails with an UnboundLocalError if the first line in the try
clause fails.  Fixed this by setting 'root = None' before each such
try clause, and adding a None check to _closeDB().  Also removed a
useless 'root = None' from the body of _closeDB().

Barry: I saw this in the saz 1.0c1 release.  Is it worth backporting?
Probably not given that we're not going to do a merge pre-1.0.

src/BTrees/tests/testBTrees.py

@@ -37,8 +37,8 @@ class Base:
         return root
 
     def _closeDB(self, root):
+        if root is not None:
             root._p_jar._db.close()
-        root = None
 
     def _delDB(self):
         for file in glob('fs_tmp__*'):
@@ -48,6 +48,7 @@ class Base:
         for i in 0, 10, 1000:
             t = self.t.__class__()
             self._populate(t, i)
+            root = None
             try:
                 root = self._getRoot()
                 root[i] = t
@@ -59,6 +60,7 @@ class Base:
 
             self._closeDB(root)
 
+            root = None
             try:
                 root = self._getRoot()
                 #XXX BTree stuff doesn't implement comparison
@@ -74,6 +76,7 @@ class Base:
         for i in 0, 10, 1000:
             t = self.t.__class__()
             self._populate(t, i)
+            root = None
             try:
                 root = self._getRoot()
                 root[i] = t
@@ -85,6 +88,7 @@ class Base:
 
             self._closeDB(root)
 
+            root = None
             try:
                 root = self._getRoot()
                 root[i]._p_changed = None