Skip over unknown boxes (#4)

* Skip over unknown boxes

If the box size is non-zero, skip over it and continue reading. Otherwise if
the box size is zero, exit the loop.

* Add comment clarifying the box skipping

src/lib.rs

@@ -83,7 +83,15 @@ fn read_boxes(f: File) -> Result<BMFF> {
             BoxType::MoofBox => {
                 start = (size as u32 - HEADER_SIZE) as u64;
             }
-            _ => break
+            _ => {
+                // Skip over unsupported boxes, but stop if the size is zero,
+                // meaning the last box has been reached.
+                if size == 0 {
+                    break;
+                } else {
+                    start = (size as u32 - HEADER_SIZE) as u64;
+                }
+            }
         };
     }
     Ok(bmff)