tcmalloc: make tc_malloc_size(NULL) return 0 instead of asserting
tcmalloc: make tc_malloc_size(NULL) return 0 instead of asserting

Although unspecified, this is consistent with GLIBC's implementation that third-party code is relying on.

BUG=118087
TEST=run all tests, run aura chrome --ui-use-gpu-process with NVIDIA driver, observe no assert.

Review URL: https://chromiumcodereview.appspot.com/9701053

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@127006 0039d316-1c4b-4281-b951-d872f2087c98

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