"-Wreturn-type" breaking the build

[apkar]

We had to disable this warning due to bad code in one place. It should be easy to reproduce by re-enabling this flag here.

[dongxinEric]

To further clarify:

• This warning/error only happens in Linux using gcc 4.9.1.
• The code can build just fine even with this flag enabled in macOS using AppleClang.

[senthil-db-expert]

@apkar We could fix this, please assign to me.
The return statements were not declared properly in some functions.