patch 9.1.1137: ins_str() is inefficient by calling STRLEN()

Problem:  ins_str() is inefficient by calling STRLLEN()
Solution: refactor ins_str() to take a length argument
          and let all callers provide the correct length
          when calling ins_str() (John Marriott)

closes: #16711

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/gui.c b/src/gui.c
index bebd51a..0b9e328 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5289,7 +5289,7 @@
 
 		    del_bytes((long)(regmatch.endp[0] - regmatch.startp[0]),
 								FALSE, FALSE);
-		    ins_str(repl_text);
+		    ins_str(repl_text, STRLEN(repl_text));
 		}
 	    }
 	    else