Assert when creating nullptr spans with a nonzero length.

According to https://en.cppreference.com/w/cpp/container/span/span (2),
it  is undefined behavior if [first, first + count) is not a valid
range, so it is standards-conforming to assert on this span.

This caught one case in GlyphRun where we were creating an invalid
span (which was apparently harmless in practice).

Change-Id: Ia0661b3bd13c2c5773492960eea11b1a294072d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/550499
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

include/core/SkSpan.h

@@ -26,6 +26,7 @@ class SkSpan {
 public:
     constexpr SkSpan() : fPtr{nullptr}, fSize{0} {}
     constexpr SkSpan(T* ptr, size_t size) : fPtr{ptr}, fSize{size} {
+        SkASSERT(ptr || size == 0);  // disallow nullptr + a nonzero size
         SkASSERT(size < kMaxSize);
     }
     template <typename U, typename = typename std::enable_if<std::is_same<const U, T>::value>::type>

src/core/SkGlyphRun.cpp

@@ -263,12 +263,13 @@ const SkGlyphRunList& SkGlyphRunBuilder::blobToGlyphRunList(
             }
         }
 
+        const uint32_t* clusters = it.clusters();
         this->makeGlyphRun(
                 font,
                 glyphIDs,
                 positions,
                 SkSpan<const char>(it.text(), it.textSize()),
-                SkSpan<const uint32_t>(it.clusters(), runSize),
+                SkSpan<const uint32_t>(clusters, clusters ? runSize : 0),
                 scaledRotations);
     }