Fix bug in SkWriter32.

If you give SkWriter32 external storage but that external storage is too small
for the first reservation you make, you'll hit an assert in debug mode.

I think the answer is to simply remove the SkASSERT.  Added a test.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/23646007

git-svn-id: http://skia.googlecode.com/svn/trunk/src@11062 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkWriter32.cpp b/core/SkWriter32.cpp
index ca21c5a..e5befba 100644
--- a/core/SkWriter32.cpp
+++ b/core/SkWriter32.cpp
@@ -61,7 +61,6 @@
         fHead = fTail = block = Block::Create(SkMax32(size, fMinSize));
         SkASSERT(0 == fWrittenBeforeLastBlock);
     } else {
-        SkASSERT(fSize > 0);
         fWrittenBeforeLastBlock = fSize;
 
         fTail = Block::Create(SkMax32(size, fMinSize));