Wrong memory allocation size for string copy at load_balancer_api.cc

[apach301]

Hi,
I found a wrong memory allocation during a string copy with Svace static analyzer.

During a string copy in GrpcLbLoadReportRequestCreate() the result of strlen is used to allocate memory, but it is not count a null terminating symbol:

grpc/src/core/load_balancing/grpclb/load_balancer_api.cc

Lines 107 to 109 in 1585097

	const size_t token_len = strlen(cur.token.get());
	char* token = reinterpret_cast<char*>(upb_Arena_Malloc(arena, token_len));
	memcpy(token, cur.token.get(), token_len);

[markdroth]

The existing code is correct. As you can see in your PR, the tests fail with the proposed change.