Fix a test warning on some linux machines

tests/DocumentTest.cpp

 #include "Test.h"
 #include "TestClassDef.h"
 
 #include "SkCanvas.h"
 #include "SkDocument.h"
 #include "SkOSFile.h"
 #include "SkStream.h"
 
 static void test_empty(skiatest::Reporter* reporter) {
     SkDynamicMemoryWStream stream;
@@ skipping 61 matching lines @@
 
     SkCanvas* canvas = doc->beginPage(100, 100);
 
     canvas->drawColor(SK_ColorRED);
     doc->endPage();
     doc->close();
 
     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);
 }
 
 static void test_close(skiatest::Reporter* reporter) {
     SkDynamicMemoryWStream stream;
     SkAutoTUnref<SkDocument> doc(SkDocument::CreatePDF(&stream));
 
     SkCanvas* canvas = doc->beginPage(100, 100);
     canvas->drawColor(SK_ColorRED);
     doc->endPage();
 
     doc->close();
 
     REPORTER_ASSERT(reporter, stream.bytesWritten() != 0);
 }
 
 DEF_TEST(document_tests, reporter) {
     test_empty(reporter);
     test_abort(reporter);
     test_abortWithFile(reporter);
     test_file(reporter);
     test_close(reporter);
 }