Fix crash when mining with empty keypool.

Since the introduction of the ScriptForMining callback, the mining
functions (setgenerate and generate) crash with an assertion failure
(due to a NULL pointer script returned) if the keypool is empty.  Fix
this by giving a proper error.

qa/rpc-tests/keypool.py

@@ -73,6 +73,21 @@ def run_test(nodes, tmpdir):
     except JSONRPCException,e:
         assert(e.error['code']==-12)
 
+    # refill keypool with three new addresses
+    nodes[0].walletpassphrase('test', 12000)
+    nodes[0].keypoolrefill(3)
+    nodes[0].walletlock()
+
+    # drain them by mining
+    nodes[0].generate(1)
+    nodes[0].generate(1)
+    nodes[0].generate(1)
+    nodes[0].generate(1)
+    try:
+        nodes[0].generate(1)
+        raise AssertionError('Keypool should be exhausted after three addesses')
+    except JSONRPCException,e:
+        assert(e.error['code']==-12)
 
 def main():
     import optparse