Initialize memory of bidi chain

This fixes an error of reading uninitialized memory, as reported by
Valgrind.

Fixes #19.

Source/SBParagraph.c

@@ -62,6 +62,9 @@ static ParagraphContextRef CreateParagraphContext(const SBBidiType *types, SBLev
         SBBidiType *fixedTypes = (SBBidiType *)(memory + offsetTypes);
 
         BidiChainInitialize(&context->bidiChain, fixedTypes, levels, fixedLinks);
+        for (SBUInteger i = 0; i < length + 2; ++i) {
+            fixedTypes[i] = SBBidiTypeNil;
+        }
         StatusStackInitialize(&context->statusStack);
         RunQueueInitialize(&context->runQueue);
         IsolatingRunInitialize(&context->isolatingRun);