SkBlockMemoryStream: correct test in getMemoryBase().

The test "nullptr == fBlockMemory->fHead->fNext" tests that
all of the data is in one block, but it is also necessary to
test "fBlockMemory->fHead" to ensure that there is a first
block with data in it.

Review URL: https://codereview.chromium.org/1374493002
2 files changed