audioflinger: Fix missing 'return' statement in 'getParameters'

Without a 'return', the code attempts to call a method on
a null thread instance.

Bug: 62918718
Test: doesn't crash when bogus audio_io_handle passed to 'getParameters'
Change-Id: I356dfd9531e0de41d869b8e29909dee5843dc795
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index d850aa9..fe5a054 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -1279,7 +1279,7 @@
         if (thread == NULL) {
             thread = (ThreadBase *)checkMmapThread_l(ioHandle);
             if (thread == NULL) {
-                String8("");
+                return String8("");
             }
         }
     }