tdf#120703 (PVS): handle failed realloc

V701 realloc() possible leak: when realloc() fails in allocating memory,
     original pointer 'm_p' is lost. Consider assigning realloc() to a
     temporary pointer.

Change-Id: Ic298927a266da4d543f3856a1c9f97ba1f6fe941
Reviewed-on: https://gerrit.libreoffice.org/62092Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>

io/source/stm/streamhelper.cxx

@@ -75,8 +75,10 @@ void MemRingBuffer::resizeBuffer( sal_Int32 nMinSize )
     }
 
     if( nNewLen != m_nBufferLen ) {
-        m_p = static_cast<sal_Int8 *>(std::realloc( m_p , nNewLen ));
-        if( !m_p ) {
+       if (auto p = static_cast<sal_Int8*>(std::realloc(m_p, nNewLen)))
+            m_p = p;
+        else
+        {
             throw css::io::BufferSizeExceededException(
                 "MemRingBuffer::resizeBuffer BufferSizeExceededException");
         }