[ruby/prism] Amend assert in pm_utf_8_codepoint(), n=0 is fine

This assert used to trip in the included test:
```
./miniruby --parser=prism -e ' "%W"\u" '
```

https://github.com/ruby/prism/commit/8c0f84db4f

1 files changed, 1 insertions, 1 deletions

diff --git a/prism/encoding.c b/prism/encoding.c
index bdb9958e97..c8e0941d1d 100644
--- a/prism/encoding.c
+++ b/prism/encoding.c
@@ -2252,7 +2252,7 @@ static const uint8_t pm_utf_8_dfa[] = {
  */
 static pm_unicode_codepoint_t
 pm_utf_8_codepoint(const uint8_t *b, ptrdiff_t n, size_t *width) {
-    assert(n >= 1);
+    assert(n >= 0);
     size_t maximum = (size_t) n;
 
     uint32_t codepoint;