Destruct USDTContext correctly in Python

src/cc/usdt/usdt.cc

@@ -414,8 +414,10 @@ void *bcc_usdt_new_frompath(const char *path) {
 }
 
 void bcc_usdt_close(void *usdt) {
-  USDT::Context *ctx = static_cast<USDT::Context *>(usdt);
-  delete ctx;
+  if (usdt) {
+    USDT::Context *ctx = static_cast<USDT::Context *>(usdt);
+    delete ctx;
+  }
 }
 
 int bcc_usdt_enable_probe(void *usdt, const char *probe_name,

src/python/bcc/usdt.py

@@ -142,6 +142,9 @@ class USDT(object):
             raise USDTException(
                     "either a pid or a binary path must be specified")
 
+    def __del__(self):
+        lib.bcc_usdt_close(self.context)
+
     def enable_probe(self, probe, fn_name):
         if lib.bcc_usdt_enable_probe(self.context, probe.encode('ascii'),
                 fn_name.encode('ascii')) != 0: