allow NULL in writeString/readString

src/core/SkWriter32.cpp

Index: src/core/SkWriter32.cpp
diff --git a/src/core/SkWriter32.cpp b/src/core/SkWriter32.cpp
index 56edd19698031ace0308c7a09bf1ad93585c600e..e41e2df0c470423c34d017bc51e324165d9ed19a 100644
--- a/src/core/SkWriter32.cpp
+++ b/src/core/SkWriter32.cpp
@@ -246,6 +246,12 @@ void SkWriter32::validate() const {
 
 const char* SkReader32::readString(size_t* outLen) {
     size_t len = this->readInt();
+    if (0xFFFF == len) {
+        if (outLen) {
+            *outLen = 0;
+        }
+        return NULL;
+    }
     const void* ptr = this->peek();
 
     // skip over teh string + '\0' and then pad to a multiple of 4
@@ -268,8 +274,12 @@ size_t SkReader32::readIntoString(SkString* copy) {
 }
 
 void SkWriter32::writeString(const char str[], size_t len) {
+    if (NULL == str) {
+        // We're already requiring len < 0xFFFF, so we can use that to mark NULL.
+        this->write32(0xFFFF);
+        return;
+    }
     if ((long)len < 0) {
-        SkASSERT(str);
         len = strlen(str);
     }
     this->write32(len);