cstr.c: nil iff terminated validly

* ext/-test-/string/cstr.c (bug_str_cstr_term_char): return nil
  only when terminated validly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49414 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/-test-/string/cstr.c

@@ -36,8 +36,9 @@ bug_str_cstr_term_char(VALUE str)
     }
     else {
 	c = rb_enc_mbc_to_codepoint(s, s + len, enc);
+	if (!c) return Qnil;
     }
-    return c ? rb_enc_uint_chr((unsigned int)c, enc) : Qnil;
+    return rb_enc_uint_chr((unsigned int)c, enc);
 }
 
 static VALUE