MPEG4Writer: Fix writing multiple NAL units in a video sample

start code is 4-byte long

Bug: 27596987
Change-Id: Ie54167f8f1b5b00679243a41e0203fdf4018f639
diff --git a/media/libstagefright/MPEG4Writer.cpp b/media/libstagefright/MPEG4Writer.cpp
index b5eb50b..e8b3dec 100644
--- a/media/libstagefright/MPEG4Writer.cpp
+++ b/media/libstagefright/MPEG4Writer.cpp
@@ -1179,7 +1179,7 @@
 
     while (getNextNALUnit(&data, &searchSize, &nextNalStart,
             &nextNalSize, true) == OK) {
-        size_t currentNalSize = nextNalStart - currentNalStart - 3 /* strip start-code */;
+        size_t currentNalSize = nextNalStart - currentNalStart - 4 /* strip start-code */;
         MediaBuffer *nalBuf = new MediaBuffer((void *)currentNalStart, currentNalSize);
         addLengthPrefixedSample_l(nalBuf);
         nalBuf->release();