fix: check for non-zero count before calling slice::from_raw_parts_mut (#686)

Fixes #681

Co-authored-by: Federico Giraud <fede1024@users.noreply.github.com>

Author: hadronzoo

src/topic_partition_list.rs

@@ -317,6 +317,9 @@ impl TopicPartitionList {
 
     /// Sets all partitions in the list to the specified offset.
     pub fn set_all_offsets(&mut self, offset: Offset) -> Result<(), KafkaError> {
+        if self.count() == 0 {
+            return Ok(());
+        }
         let slice = unsafe { slice::from_raw_parts_mut(self.ptr.elems, self.count()) };
         for elem_ptr in slice {
             let mut elem = TopicPartitionListElem::from_ptr(self, &mut *elem_ptr);
@@ -327,8 +330,11 @@ impl TopicPartitionList {
 
     /// Returns all the elements of the list.
     pub fn elements(&self) -> Vec<TopicPartitionListElem<'_>> {
+        let mut vec = Vec::with_capacity(self.count());
+        if self.count() == 0 {
+            return vec;
+        }
         let slice = unsafe { slice::from_raw_parts_mut(self.ptr.elems, self.count()) };
-        let mut vec = Vec::with_capacity(slice.len());
         for elem_ptr in slice {
             vec.push(TopicPartitionListElem::from_ptr(self, &mut *elem_ptr));
         }
@@ -337,8 +343,11 @@ impl TopicPartitionList {
 
     /// Returns all the elements of the list that belong to the specified topic.
     pub fn elements_for_topic<'a>(&'a self, topic: &str) -> Vec<TopicPartitionListElem<'a>> {
+        let mut vec = Vec::with_capacity(self.count());
+        if self.count() == 0 {
+            return vec;
+        }
         let slice = unsafe { slice::from_raw_parts_mut(self.ptr.elems, self.count()) };
-        let mut vec = Vec::with_capacity(slice.len());
         for elem_ptr in slice {
             let tp = TopicPartitionListElem::from_ptr(self, &mut *elem_ptr);
             if tp.topic() == topic {