Fix integer underflow in ESDS processing

Several arithmetic operations within parseESDescriptor could underflow, leading
to an out-of-bounds read operation. Ensure that subtractions from 'size' do not
cause it to wrap around.

Bug: 20139950

(cherry picked from commit 07c0f59)

Change-Id: I377d21051e07ca654ea1f7037120429d3f71924a

media/libstagefright/ESDS.cpp

@@ -136,6 +136,8 @@ status_t ESDS::parseESDescriptor(size_t offset, size_t size) {
     --size;
 
     if (streamDependenceFlag) {
+        if (size < 2)
+            return ERROR_MALFORMED;
         offset += 2;
         size -= 2;
     }
@@ -145,11 +147,15 @@ status_t ESDS::parseESDescriptor(size_t offset, size_t size) {
             return ERROR_MALFORMED;
         }
         unsigned URLlength = mData[offset];
+        if (URLlength >= size)
+            return ERROR_MALFORMED;
         offset += URLlength + 1;
         size -= URLlength + 1;
     }
 
     if (OCRstreamFlag) {
+        if (size < 2)
+            return ERROR_MALFORMED;
         offset += 2;
         size -= 2;