IOBufTest.cpp: fix a test for llvm15

Summary:
Avoid this: (full log P847931899):
  $ buck2 test -c fbcode.platform010_clang=15 folly/io/test:iobuf_test -- --exact 'folly/io/test:iobuf_test - IOBuf.computeChainCapacityOfZeroSizeIOBuf'
  ...
    fbcode/folly/io/test/IOBufTest.cpp:1762:16: runtime error: variable length array bound evaluates to non-positive value 0

Reviewed By: bcardosolopes, luciang

Differential Revision: D50027080

fbshipit-source-id: 66d0a4975b5484b72920887fefac1bcd6b84b4b7

folly/io/test/IOBufTest.cpp

@@ -1759,7 +1759,7 @@ TEST(IOBuf, FreeFn) {
 // Compute the chained capacity of a single non-chained IOBuf of capacity zero
 TEST(IOBuf, computeChainCapacityOfZeroSizeIOBuf) {
   size_t size = 0;
-  uint8_t data[size];
+  uint8_t* data = nullptr;
 
   // Create buffer of capacity 0
   unique_ptr<IOBuf> buf(IOBuf::wrapBuffer(data, size));