patch 8.1.0593: illegal memory access in libvterm test

Problem:    Illegal memory access in libvterm test.
Solution:   Fix off-by-one error.
diff --git a/src/libvterm/Makefile b/src/libvterm/Makefile
index 6eacab7..179ee10 100644
--- a/src/libvterm/Makefile
+++ b/src/libvterm/Makefile
@@ -52,6 +52,9 @@
 MANDIR=$(PREFIX)/share/man
 MAN3DIR=$(MANDIR)/man3
 
+# Uncomment to check for memory access errors with valgrind.
+# VALGRIND=1
+
 all: $(LIBRARY) $(BINFILES)
 
 $(LIBRARY): $(OBJECTS)