Fix a crash in SkParsePath::FromSVGString

BUG=skia:3491

Review URL: https://codereview.chromium.org/1515193002
diff --git a/src/utils/SkParsePath.cpp b/src/utils/SkParsePath.cpp
index 3fecb46..3eb9e1e 100644
--- a/src/utils/SkParsePath.cpp
+++ b/src/utils/SkParsePath.cpp
@@ -77,6 +77,10 @@
     char previousOp = '\0';
     bool relative = false;
     for (;;) {
+        if (!data) {
+            // Truncated data
+            return false;
+        }
         data = skip_ws(data);
         if (data[0] == '\0') {
             break;