patch 9.1.0152: Coverity complains about ignoring return value

Problem:  Coverity complains about ignoring return value of win_split()
          (after v9.1.150)
Solution: Check if win_split() failed, add winfixbuf.res to Makefile
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index d365dfc..c4403e6 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -558,6 +558,7 @@
 	test_window_cmd.res \
 	test_window_id.res \
 	test_windows_home.res \
+	test_winfixbuf.res \
 	test_wordcount.res \
 	test_writefile.res \
 	test_xxd.res \