Return value of function which returns null is dereferenced without checking

Defect 76371 (STATIC_C)
Checker NULL_RETURNS (subcategory none)
File: /src/mongoc/mongoc-collection.c
Function mongoc_collection_create_index
/src/mongoc/mongoc-collection.c, line: 821
Assigning: "alloc_name" = null return value from "mongoc_collection_keys_to_index_string".

          alloc_name = mongoc_collection_keys_to_index_string (keys);

/src/mongoc/mongoc-collection.c, line: 822
Assigning: "name" = "alloc_name". Both pointers are now null.

          name = alloc_name;

/src/mongoc/mongoc-collection.c, line: 832
Dereferencing a pointer that might be null "name" when calling "strlen".

       BSON_APPEND_UTF8 (&doc, "name", name);