utf8 fixes

src/main.c

@@ -527,6 +527,12 @@ void string_test_utf8(void)
     Str str3 = S("1Å 2\xc6" "3\xc3");
     TRACEF("str3:\n");
     print_runes(str3);
+
+    const char rune_error[] = {0xEF, 0xBF, 0xBD, 0x00} ; // RUNE_ERROR encoded in utf-8
+    Str str4 = S(rune_error);
+    TRACEF("str4:\n");
+    print_runes(str4);  // this should be a valid code point
+
 }
 
 int main(int argc, char *argv[])