patch 8.2.0967: unnecessary type casts for vim_strnsave()

Problem:    Unnecessary type casts for vim_strnsave().
Solution:   Remove the type casts.
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index a55750b..444b446 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -7103,8 +7103,7 @@
 
 		if (lpos->start != NULL && lpos->end != NULL)
 		    re_extmatch_out->matches[i] =
-			    vim_strnsave(lpos->start,
-					      (int)(lpos->end - lpos->start));
+			    vim_strnsave(lpos->start, lpos->end - lpos->start);
 	    }
 	}
     }