Fix a test warning on some linux machines

R=sugoi@google.com

Review URL: https://codereview.chromium.org/34803004

git-svn-id: http://skia.googlecode.com/svn/trunk@11910 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/DocumentTest.cpp b/tests/DocumentTest.cpp
index 9ca7881..1838687 100644
--- a/tests/DocumentTest.cpp
+++ b/tests/DocumentTest.cpp
@@ -79,7 +79,7 @@
     FILE* file = fopen(path.c_str(), "r");
     REPORTER_ASSERT(reporter, file != NULL);
     char header[100];
-    fread(header, 4, 1, file);
+    REPORTER_ASSERT(reporter, fread(header, 4, 1, file) != 0);
     REPORTER_ASSERT(reporter, strncmp(header, "%PDF", 4) == 0);
     fclose(file);
 }