Repair temp filename bug in PNaCl driver.
Repair temp filename bug in PNaCl driver.

In certain cases (where the output filename was a prefix of the input filename)
this would cause temporary file names to be incorrect.
Review URL: http://codereview.chromium.org/6833024

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@4863 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client