Fix valgrind issue in SkBase64Test.cpp

This addresses the valgrind issue:

Conditional jump or move depends on uninitialised value(s)
   at 0x636528: SkBase64::decode(void const*, unsigned long, bool) (SkBase64.cpp:63)
   by 0x6367AC: SkBase64::decode(char const*, unsigned long) (SkBase64.cpp:157)
   by 0x4865F2: SkBase64Test(skiatest::Reporter*) (SkBase64Test.cpp:24)
   by 0x486703: skiatest::SkBase64TestClass::onRun(skiatest::Reporter*) (SkBase64Test.cpp:12)
   by 0x48E48A: skiatest::Test::run() (Test.cpp:105)

introduced in r13254 (Port SkBase64 test to our test driver - https://codereview.chromium.org/132233060)

TBR=tfarina@chromium.org

[robertphillips, 2014-02-01 15:45:37]

committed as r13276

[tfarina, 2014-02-01 15:46:21]

lgtm

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp
File tests/SkBase64Test.cpp (right):

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp#newco...
tests/SkBase64Test.cpp:14: for (int index = 0; index < 256; ++index) {
does pre-increment makes any difference here, or was this a pure-cosmetic
change?

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp#newco...
tests/SkBase64Test.cpp:23: src[encodeLength] = '\0';
looks like I removed this when I used SkAutoMalloc (which does not have
operator[]) and forgot to add it back when I switched to SkAutoTMalloc.

[robertphillips, 2014-02-01 15:50:23]

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp
File tests/SkBase64Test.cpp (right):

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp#newco...
tests/SkBase64Test.cpp:14: for (int index = 0; index < 256; ++index) {
In this case it doesn't make any performance difference and is more of a style
thing. For actual class based iterators pre vs. post increment/decrement could
make a difference.

https://codereview.chromium.org/152153002/diff/1/tests/SkBase64Test.cpp#newco...
tests/SkBase64Test.cpp:23: src[encodeLength] = '\0';
Yep - np. The recent noise on the valgrind bot hid this yesterday.