patch 9.1.1334: Coverity complains about unchecked return value

Problem:  Coverity complains about unchecked return value
Solution: cast return value to (void)

Fixes: Coverity issue 1646574
related: #17189

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/session.c b/src/session.c
index eea57a2..56d368a 100644
--- a/src/session.c
+++ b/src/session.c
@@ -1072,7 +1072,7 @@
     if (fname == NULL)
 	return;
 
-    do_source(fname, FALSE, DOSO_NONE, NULL);
+    (void)do_source(fname, FALSE, DOSO_NONE, NULL);
     vim_free(fname);
 }