Rev-parse: proper error checking.

src/revparse.c

@@ -169,10 +169,9 @@ static int walk_ref_history(git_object **out, git_repository *repo, const char *
          giterr_set(GITERR_INVALID, "Invalid reflogspec %s", reflogspec);
          return GIT_ERROR;
       }
-      git_reference_lookup(&ref, repo, "HEAD");
-      git_reflog_read(&reflog, ref);
-      git_reference_free(ref);
 
+      if (!git_reference_lookup(&ref, repo, "HEAD")) {
+         if (!git_reflog_read(&reflog, ref)) {
             regex_error = regcomp(&regex, "checkout: moving from (.*) to .*", REG_EXTENDED);
             if (regex_error != 0) {
                giterr_set_regex(&regex, regex_error);
@@ -196,6 +195,9 @@ static int walk_ref_history(git_object **out, git_repository *repo, const char *
                }
                regfree(&regex);
             }
+         }
+         git_reference_free(ref);
+      }
    } else {
       git_buf datebuf = GIT_BUF_INIT;
       git_buf_put(&datebuf, reflogspec+2, reflogspeclen-3);