folly: fix IOBufCursorTest

Summary:
```
folly/folly/io/test/IOBufCursorTest.cpp:212:3:
folly/folly/io/Cursor.h:411:13: error: ‘void* memcpy(void*, const void
*, size_t)’ forming offset [13, 20] is out of the bounds [0, 12] of object ‘buf’ with type ‘char [12]’ [-Werror=array-bounds]
       memcpy(buf, data(), len);
       ~~~~~~^~~~~~~~~~~~~~~~~~
```

Reviewed By: yfeldblum

Differential Revision: D14702563

fbshipit-source-id: 3697a021d71bccfae9d8833b42a0c25e916b7a78

folly/io/test/IOBufCursorTest.cpp

@@ -203,7 +203,7 @@ TEST(IOBuf, PullAndPeek) {
   EXPECT_EQ(3, iobuf1->countChainElements());
   EXPECT_EQ(11, iobuf1->computeChainDataLength());
 
-  char buf[12];
+  char buf[20];
   memset(buf, 0, sizeof(buf));
   Cursor(iobuf1.get()).pull(buf, 11);
   EXPECT_EQ("hello world", std::string(buf));