libdpkg: Fix undefined behavior in varbuf functions

While varbuf_grow() handles 0 sizes fine, we do a check before calling
it to avoid the duplicate checks.

Reported-by: KOLANICH <kolan_n@mail.ru>
Warned-by: gcc ASAN

lib/dpkg/varbuf.c

@@ -40,6 +40,8 @@ varbuf_add_char(struct varbuf *v, int c)
 void
 varbuf_dup_char(struct varbuf *v, int c, size_t n)
 {
+  if (n == 0)
+    return;
   varbuf_grow(v, n);
   memset(v->buf + v->used, c, n);
   v->used += n;
@@ -95,6 +97,8 @@ varbuf_vprintf(struct varbuf *v, const char *fmt, va_list args)
 void
 varbuf_add_buf(struct varbuf *v, const void *s, size_t size)
 {
+  if (size == 0)
+    return;
   varbuf_grow(v, size);
   memcpy(v->buf + v->used, s, size);
   v->used += size;