fix bug causing skipped mutation when part size is 0

libafl/src/mutators/multi.rs

@@ -299,6 +299,10 @@ where
             drop(other_testcase);
             let size = part.bytes().len();
 
+            if size == 0 {  // Workaround for a bug where parts are empty. The origin remains unknown.
+                return Ok(MutationResult::Skipped);
+            }
+
             let target = state.rand_mut().below(size);
             let range = rand_range(state, other_size, min(other_size, size - target));