Bug 1574392: fix for crash caused by a zero length textblob r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D42347

--HG--
extra : moz-landing-system : lando

layout/painting/crashtests/1574392.html

@@ -0,0 +1 @@
+<u>🏴󠁵󠁳󠁣󠁡󠁿</u>

layout/painting/crashtests/crashtests.list

@@ -21,4 +21,4 @@ load 1547420-1.html
 load 1549909.html
 asserts(6) load 1551389-1.html # bug 847368
 asserts(0-2) load 1555819-1.html
-
+load 1574392.html

layout/painting/nsCSSRendering.cpp

@@ -3877,7 +3877,10 @@ static sk_sp<const SkTextBlob> CreateTextBlob(
   // allocate space for the run buffer, then fill it with the glyphs
   uint32_t len =
       CountAllGlyphs(aTextRun, aCompressedGlyph, aStringStart, aStringEnd);
-  MOZ_ASSERT(len > 0, "there must be at least one glyph for skip ink");
+  if (len <= 0) {
+    return nullptr;
+  }
+
   SkTextBlobBuilder builder;
   const SkTextBlobBuilder::RunBuffer& run = builder.allocRunPos(aFont, len);
 
@@ -4116,6 +4119,10 @@ void nsCSSRendering::PaintDecorationLine(
                        iter.GetStringStart(), iter.GetStringEnd(),
                        (float)appUnitsPerDevPixel, textPos, spacingOffset);
 
+    if (!textBlob) {
+      continue;
+    }
+
     if (textRun->UseCenterBaseline()) {
       // writing modes that use a center baseline need to be adjusted on a
       // font-by-font basis since Skia lines up the text on a alphabetic