update_client: Check string length before calling front().

(cherry picked from commit 6581c6b)

Fixed: 1486316
Change-Id: I7be04ea0c8e040b5a67364925fc06d4ee9167242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4892838
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1201617}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4902697
Cr-Commit-Position: refs/branch-heads/5993@{#978}
Cr-Branched-From: 5113507-refs/heads/main@{#1192594}

components/update_client/protocol_parser_json.cc

@@ -196,7 +196,7 @@ bool ParseUpdateCheck(const base::Value& updatecheck_node_val,
   const base::Value::Dict& updatecheck_node = updatecheck_node_val.GetDict();
 
   for (auto kv : updatecheck_node) {
-    if (kv.first.front() == '_' && kv.second.is_string()) {
+    if (!kv.first.empty() && kv.first.front() == '_' && kv.second.is_string()) {
       result->custom_attributes[kv.first] = kv.second.GetString();
     }
   }

components/update_client/protocol_parser_json_unittest.cc

@@ -393,6 +393,10 @@ const char* kJSONCustomAttributes = R"()]}'
    ]
   }})";
 
+const char* kBadJSONBadAppIdNoNewlinesBadUCKey =
+    R"()]}'{"response":{"app":[{"appid":";","updatecheck":{"":1}}],)"
+    R"("protocol":"3.1"}})";
+
 TEST(UpdateClientProtocolParserJSONTest, Parse) {
   const auto parser = std::make_unique<ProtocolParserJSON>();
 
@@ -610,4 +614,9 @@ TEST(UpdateClientProtocolParserJSONTest, ParseAttrs) {
   }
 }
 
+TEST(UpdateClientProtocolParserJSONTest, ParseBadJSONNoCrash) {
+  const auto parser = std::make_unique<ProtocolParserJSON>();
+  EXPECT_TRUE(parser->Parse(kBadJSONBadAppIdNoNewlinesBadUCKey));
+}
+
 }  // namespace update_client