QDecompressHelper: fix compilation with GCC 9

GCC 9 doesn't accept [[unlikely]] between the (condition) and the
compound-statement of an if, in both C++17 and C++20 modes:

  qdecompresshelper.cpp: In member function ‘qsizetype QDecompressHelper::readZLib(char*, qsizetype)’:
  qcompilerdetection.h:1048:31: error: attributes at the beginning of statement are ignored [-Werror=attributes]
   1048 | #    define Q_UNLIKELY_BRANCH [[unlikely]]
        |                               ^~~~~~~~~~~~
  qdecompresshelper.cpp:597:54: note: in expansion of macro ‘Q_UNLIKELY_BRANCH’
    597 |         if (ret == Z_DATA_ERROR && !triedRawDeflate) Q_UNLIKELY_BRANCH {
        |                                                      ^~~~~~~~~~~~~~~~~

See also https://stackoverflow.com/questions/51797959/how-to-use-c20s-likely-unlikely-attribute-in-if-else-statement

Put it into the compound-statement instead, then GCC 9 accepts it. The
two are equivalent, because [[likely]] marks a path, and there is no
selection statement between the two positions.

Amends 5ae84d0.

Pick-to: 6.9
Change-Id: Iac1970219c98a1c26e450dfe6bad6583e4d32c29
Reviewed-by: Thiago Macieira <[email protected]>

src/network/access/qdecompresshelper.cpp

@@ -594,7 +594,7 @@ qsizetype QDecompressHelper::readZLib(char *data, const qsizetype maxSize)
         // also an error.
         // in the case where we get Z_DATA_ERROR this could be because we received raw deflate
         // compressed data.
-        if (ret == Z_DATA_ERROR && !triedRawDeflate) Q_UNLIKELY_BRANCH {
+        if (ret == Z_DATA_ERROR && !triedRawDeflate) { Q_UNLIKELY_BRANCH
             inflateEnd(inflateStream);
             triedRawDeflate = true;
             inflateStream->zalloc = Z_NULL;
@@ -768,7 +768,7 @@ qsizetype QDecompressHelper::readZstandard(char *data, const qsizetype maxSize)
     qsizetype bytesDecoded = 0;
     while (outBuf.pos < outBuf.size && (inBuf.pos < inBuf.size || decoderHasData)) {
         size_t retValue = ZSTD_decompressStream(zstdStream, &outBuf, &inBuf);
-        if (ZSTD_isError(retValue)) Q_UNLIKELY_BRANCH {
+        if (ZSTD_isError(retValue)) { Q_UNLIKELY_BRANCH
             errorStr = QCoreApplication::translate("QHttp", "ZStandard error: %1")
                             .arg(QUtf8StringView{ZSTD_getErrorName(retValue)});
                 return -1;