fix compilation bug on gcc 8.1.0

[recolic]

I tried git clone https://github.com/grpc/grpc && cd grpc && make -j4 with gcc 8.1.0(ArchLinux) and I got many compilation bugs. So I tried to fix them.

[thelinuxfoundation]

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[nicolasnoble]

@recolic thanks for the contribution, and it looks good to me, but before we can accept it, we need you to sign the CLA as the bot mentions.

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[grpc-testing]

[microbenchmarks] No significant performance differences

[recolic]

@nicolasnoble CLA signed......

[markdroth]

Cleaning up old PRs. If this is still something you want to pursue, please re-open.

[recolic]

@markdroth Is this issue resolved?

[markdroth]

Not as far as I know. If you'd still like to pursue it, please reopen the PR and make sure there are no merge conflicts with master, and we can take a look.