MPEG4Writer: Initialize mime to NULL in addSource

mime was uninitialized before querying from format.
If format doesn't contain mime, then this remained
uninitialized

Test: atest android.mediav2.cts.MuxerUnitTest
Bug: 148916777

Change-Id: I1375dd9e905fe8ee21f6cff9605db0e97114d0fa
diff --git a/media/libstagefright/MPEG4Writer.cpp b/media/libstagefright/MPEG4Writer.cpp
index 128ce8b..258bed8 100644
--- a/media/libstagefright/MPEG4Writer.cpp
+++ b/media/libstagefright/MPEG4Writer.cpp
@@ -613,7 +613,7 @@
 
     CHECK(source.get() != NULL);
 
-    const char *mime;
+    const char *mime = NULL;
     sp<MetaData> meta = source->getFormat();
     meta->findCString(kKeyMIMEType, &mime);