bash: fix CVE-2014-7186 and CVE-2014-7187

Sync up with upstream gentoo, pulling in the final version of the patch
for these issues. This is functionally equivalent to 4.2_p51 but
upstream hasn't officially announced that version yet it seems.
Merge pull request #893 from marineam/bash50

bash: fix CVE-2014-7186 and CVE-2014-7187