valgrind: Conditional jump or move depends on uninitialised value

 Conditional jump or move depends on uninitialised value(s)
    at 0x4C3535E: strstr (vg_replace_strmem.c:1623)
    by 0x4FCD9E59: SmFilterDetect::detect(com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue>&) (smdetect.cxx:102)
    by 0x41A1A774: filter::config::TypeDetection::impl_askDetectService(rtl::OUString const&, utl::MediaDescriptor&) (typedetection.cxx:1038)

Change-Id: I2f98fd3f7799f34cb5bd9242379ce6440645034d
Reviewed-on: https://gerrit.libreoffice.org/42381Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

starmath/source/smdetect.cxx

@@ -91,12 +91,12 @@ OUString SAL_CALL SmFilterDetect::detect( Sequence< PropertyValue >& lDescriptor
         // stuff I hope?
         static const sal_uInt16 nBufferSize = 200;
         char aBuffer[nBufferSize+1];
-        aBuffer[nBufferSize] = 0;
         pInStrm->Seek( STREAM_SEEK_TO_BEGIN );
         pInStrm->StartReadingUnicodeText( RTL_TEXTENCODING_DONTKNOW ); // avoid BOM marker
         auto nBytesRead = pInStrm->ReadBytes( aBuffer, nBufferSize );
         if (nBytesRead >= 6)
         {
+            aBuffer[nBytesRead] = 0;
             bool bIsMathType = false;
             if (0 == strncmp( "<?xml", aBuffer, 5))
                 bIsMathType = (strstr( aBuffer, "<math>" ) ||