Don't call std::string::back() on empty strings

Found with _LIBCPP_DEBUG=0.

Bug: 923166
Change-Id: Ifdb0be8031bc95f05ff3d67876407569bac0e106
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891235
Commit-Queue: David Benjamin <davidben@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711037}

net/dns/host_resolver_manager.cc

@@ -162,7 +162,7 @@ bool ResemblesMulticastDNSName(const std::string& hostname) {
   const char kSuffix[] = ".local.";
   const size_t kSuffixLen = sizeof(kSuffix) - 1;
   const size_t kSuffixLenTrimmed = kSuffixLen - 1;
-  if (hostname.back() == '.') {
+  if (!hostname.empty() && hostname.back() == '.') {
     return hostname.size() > kSuffixLen &&
            !hostname.compare(hostname.size() - kSuffixLen, kSuffixLen, kSuffix);
   }