[Mac] Puts a NULL check in case a native plugin library cannot be loaded (.dll on mac, for example).
[Mac] Puts a NULL check in case a native plugin library cannot be loaded (.dll on mac, for example).

TEST=Load an extension that requires a .dll or other platform-specific file and make sure it does not crash the browser.
BUG=29584
Review URL: http://codereview.chromium.org/492012

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@34308 0039d316-1c4b-4281-b951-d872f2087c98

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