patch 9.0.1241: Coverity warns for not checking function return value

Problem:    Coverity warns for not checking function return value.
Solution:   Explicitly ignore the return value.
diff --git a/src/userfunc.c b/src/userfunc.c
index 3ff49c9..77a9fc2 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -192,7 +192,7 @@
 		&& *eap->cmdlinep == ((char_u **)lines_to_free->ga_data)
 						   [lines_to_free->ga_len - 1])
 	    *eap->cmdlinep = theline;
-	ga_add_string(lines_to_free, theline);
+	(void)ga_add_string(lines_to_free, theline);
     }
 
     return theline;