Changed precondition for edge case in serial_merge to prevent asserti…

…on error (#622)

* added std::min to ensure no out of bound acess

* fixed typo keys->keys1

* updated changelog

* reverted std::min

* implemented suggested logic

CHANGELOG.md

@@ -37,6 +37,7 @@ Documentation for rocPRIM is available at
   * device merge,
   * device partial sort, and/or
   * device sort (merge sort).
+* Fixed an issue where on certain inputs to block_sort_merge, device_merge_sort_merge_path, device_merge, and warp_sort_stable  would cause an assertion error during its call to serial_merge
 
 ### Deprecations
 

rocprim/include/rocprim/detail/merge_path.hpp

@@ -100,8 +100,8 @@ void serial_merge(KeyType*                keys_shared,
                   OutputFunction          output_function)
 {
     // Pre condition, we're including some edge cases too.
-    assert(range.begin1 <= range.end1);
-    assert(range.begin2 <= range.end2);
+    if (!AllowUnsafe && range.begin1 > range.end1 && range.begin2 > range.end2)
+        return; // don't do anything, we have invalid inputs  
 
     // More descriptive names for ranges:
     auto       idx_a = range.begin1;