utils/IvfIterator: fix expected header length

A header is made of a u64 and u32 and is thus 12 bytes.
diff --git a/src/utils.rs b/src/utils.rs
index 2040a66..8cee243 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -54,7 +54,7 @@
 
     fn next(&mut self) -> Option<Self::Item> {
         // Make sure we have a header.
-        if self.cursor.remaining() < 6 {
+        if self.cursor.remaining() < 12 {
             return None;
         }