Use known length to pre-initialize buffer

... and drop now-obsolete conditional include, which is included
unconditionally since commit 4c68c4b93c
  Author Stephan Bergmann <sbergman@redhat.com>
  Date   Tue Mar 01 08:23:31 2022 +0100

    misaligned-pointer-use

Change nCount argument to sal_Int32, since that's what is used for
string lengths. The call sites use sal_uInt16 and sal_uInt8 as the
argument value.

Change-Id: Ie2a9791f3ab9565590cb4f9c42d44f6eeff29745
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130754
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

sw/source/filter/ww8/WW8Sttbf.cxx

@@ -24,15 +24,12 @@
 #include "WW8Sttbf.hxx"
 #include <osl/endian.h>
 #include <o3tl/make_shared.hxx>
+#include <o3tl/safeint.hxx>
 #include <rtl/ustrbuf.hxx>
 #include <tools/stream.hxx>
 #include <sal/log.hxx>
 #include <osl/diagnose.h>
 
-#ifdef OSL_BIGENDIAN
-#include <rtl/ustrbuf.hxx>
-#endif
-
 namespace ww8
 {
     WW8Struct::WW8Struct(SvStream& rSt, sal_uInt32 nPos, sal_uInt32 nSize)
@@ -71,7 +68,7 @@ namespace ww8
     }
 
     OUString WW8Struct::getUString(sal_uInt32 nOffset,
-                                          sal_uInt32 nCount)
+                                          sal_Int32 nCount)
     {
         OUString aResult;
 
@@ -82,10 +79,10 @@ namespace ww8
             if (nStartOff >= mn_size)
                 return aResult;
             sal_uInt32 nAvailable = (mn_size - nStartOff)/sizeof(sal_Unicode);
-            if (nCount > nAvailable)
+            if (o3tl::make_unsigned(nCount) > nAvailable)
                 nCount = nAvailable;
-            OUStringBuffer aBuf;
-            for (sal_uInt32 i = 0; i < nCount; ++i)
+            OUStringBuffer aBuf(nCount);
+            for (sal_Int32 i = 0; i < nCount; ++i)
                 aBuf.append(static_cast<sal_Unicode>(getU16(nStartOff+i*2)));
             aResult = aBuf.makeStringAndClear();
         }

sw/source/filter/ww8/WW8Sttbf.hxx

@@ -47,7 +47,7 @@ namespace ww8
         sal_uInt16 getU16(sal_uInt32 nOffset)
         { return getU8(nOffset) + (getU8(nOffset + 1) << 8); }
 
-        OUString getUString(sal_uInt32 nOffset, sal_uInt32 nCount);
+        OUString getUString(sal_uInt32 nOffset, sal_Int32 nCount);
     };
 
     template <class T>