AudioFlinger: add check for NULL mInput in MMAP Thread

exitStandby() was crashing if headphones were
unplugged between the open and start of a MMAP stream.

Bug: 120564294
Test: see repro steps in bug
Change-Id: Ic42743682ffeb48f2869eaa90581b4e4f7fcb1ab
diff --git a/services/audioflinger/Threads.cpp b/services/audioflinger/Threads.cpp
index 45d3a06..27aec9e 100644
--- a/services/audioflinger/Threads.cpp
+++ b/services/audioflinger/Threads.cpp
@@ -9146,7 +9146,13 @@
 
 status_t AudioFlinger::MmapCaptureThread::exitStandby()
 {
-    mInput->stream->setGain(1.0f);
+    {
+        // mInput might have been cleared by clearInput()
+        Mutex::Autolock _l(mLock);
+        if (mInput != nullptr && mInput->stream != nullptr) {
+            mInput->stream->setGain(1.0f);
+        }
+    }
     return MmapThread::exitStandby();
 }