Make SkROBuffer::Iter::size() work when exhausted

According to the documentation, this method will return 0 when the Iter
is exhausted. Prior to this CL, it crashes instead.

Prevent a crash with a null fHead, and add a test to verify the
behavior.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1574603002

Review URL: https://codereview.chromium.org/1574603002

src/core/SkRWBuffer.cpp

@@ -150,6 +150,9 @@ const void* SkROBuffer::Iter::data() const {
 }
 
 size_t SkROBuffer::Iter::size() const {
+    if (!fBlock) {
+        return 0;
+    }
     return SkTMin(fBlock->fUsed, fRemaining);
 }
 

tests/DataRefTest.cpp

@@ -313,3 +313,18 @@ DEF_TEST(RWBuffer, reporter) {
         delete streams[i];
     }
 }
+
+// Tests that it is safe to call SkROBuffer::Iter::size() when exhausted.
+DEF_TEST(RWBuffer_size, r) {
+    SkRWBuffer buffer;
+    buffer.append(gABC, 26);
+
+    SkAutoTUnref<SkROBuffer> roBuffer(buffer.newRBufferSnapshot());
+    SkROBuffer::Iter iter(roBuffer);
+    REPORTER_ASSERT(r, iter.data());
+    REPORTER_ASSERT(r, iter.size() == 26);
+
+    // There is only one block in this buffer.
+    REPORTER_ASSERT(r, !iter.next());
+    REPORTER_ASSERT(r, 0 == iter.size());
+}