When the upstream request to httpbingo.org transiently fails, cli.Get()
returns nullptr and the next line dereferences it (res->status / res->body),
producing a SEGV in std::string::begin() under ASan. Sibling templates in
the same file already use ASSERT_TRUE(res != nullptr); apply the same
guard to the four Get() call sites in KeepAliveTest so a flaky network
turns into a clean test failure instead of a crash.
a1fdc07f34
