RFR 8014431 : cleanup warnings indicated by the -Wunused-value compiler option on linux

[Calvin Cheung]

This change is for fixing the errors resulting from enabling the
-Wunused-value compiler option on linux.

The patch was initiated by Jeremy (jeremymanson at google.com).
More changes are needed for it to pass the build.

Tests performed:
    jprt
    jtreg on linux_x64
    vm.quick on linux_x64

Webrev: http://cr.openjdk.java.net/~ccheung/8014431/webrev/
Bug: http://bugs.sun.com/view_bug.do?bug_id=8014431

thanks,
Calvin