stagefright: avoid buffer overflow in base64 decoder

Bug: 62673128
Change-Id: Id5f04b772aaca3184879bd5bca453ad9e82c7f94
(cherry picked from commit 5e96386ab7a5391185f6b3ed9ea06f3e23ed253b)

media/libstagefright/foundation/base64.cpp

diff --git a/media/libstagefright/foundation/base64.cpp b/media/libstagefright/foundation/base64.cpp
index 7da7db9..cc89064 100644
--- a/media/libstagefright/foundation/base64.cpp
+++ b/media/libstagefright/foundation/base64.cpp
@@ -78,8 +78,7 @@
         accum = (accum << 6) | value;
 
         if (((i + 1) % 4) == 0) {
-            out[j++] = (accum >> 16);
-
+            if (j < outLen) { out[j++] = (accum >> 16); }
             if (j < outLen) { out[j++] = (accum >> 8) & 0xff; }
             if (j < outLen) { out[j++] = accum & 0xff; }