Assign pointer with null after use

Assign pointer with null after use.

CRs-Fixed: 3713074
Change-Id: I08a1b3b95ec5d0f7f61ff9613b2e46acfa7e7ce1
diff --git a/jni/android_hardware_fm.cpp b/jni/android_hardware_fm.cpp
index 76490ee..6809a27 100644
--- a/jni/android_hardware_fm.cpp
+++ b/jni/android_hardware_fm.cpp
@@ -486,6 +486,7 @@
             return;
         }
         vm->DetachCurrentThread();
+        mCallbackEnv = NULL;
     }
 }