Properly handle script teardown.

If libbcc optimizes away a global variable, we need to make sure that
RenderScript's ~Context() doesn't attempt to clear away the slot's NULL
pointer that denotes this.

Change-Id: I77d0f740ab333338e53db481e4a3eca338aba411

libs/rs/driver/rsdBcc.cpp

@@ -522,7 +522,11 @@ void rsdScriptDestroy(const Context *dc, Script *script) {
     if (drv->mFieldAddress) {
         for (size_t ct=0; ct < drv->mFieldCount; ct++) {
             if (drv->mFieldIsObject[ct]) {
-                rsiClearObject((ObjectBase **)&drv->mFieldAddress[ct]);
+                // The field address can be NULL if the script-side has
+                // optimized the corresponding global variable away.
+                if (drv->mFieldAddress[ct]) {
+                    rsiClearObject((ObjectBase **)drv->mFieldAddress[ct]);
+                }
             }
         }
         delete [] drv->mFieldAddress;