Use proper sign-ness

[kgizdov]

The following code is problematic:

xrootd/src/XrdCl/XrdClParallelOperation.hh

Lines 92 to 99 in 4ceda7c

	for( int i = 0; i < pipelines.size(); i++ )
	{
	oss << pipelines[i]->ToString();
	if( i != pipelines.size() - 1 )
	{
	oss << " && ";
	}
	}

Not to mention it fails the tests with current config:

/usr/bin/cc -pthread -DNDEBUG -g -fwrapv -O3 -Wall --std=c++0x -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt -D_FORTIFY_SOURCE=2 -fPIC -I/build/xrootd/src/xrootd-4.9.0/src -I/build/xrootd/src/build/src -I/usr/include/python3.7m -c /build/xrootd/src/xrootd-4.9.0/bindings/python/src/PyXRootDFileSystem.cc -o build/temp.linux-x86_64-3.7/build/xrootd/src/xrootd-4.9.0/bindings/python/src/PyXRootDFileSystem.o -g
In file included from /build/xrootd/src/xrootd-4.9.0/tests/XrdClTests/OperationsWorkflowTest.cc:31:
/build/xrootd/src/xrootd-4.9.0/src/XrdCl/XrdClParallelOperation.hh: In instantiation of ‘std::__cxx11::string XrdCl::ParallelOperation<<anonymous> >::ToString() [with bool HasHndl = true; std::__cxx11::string = std::__cxx11::basic_string<char>]’:
/build/xrootd/src/xrootd-4.9.0/src/XrdCl/XrdClParallelOperation.hh:88:19:   required from here
/build/xrootd/src/xrootd-4.9.0/src/XrdCl/XrdClParallelOperation.hh:92:27: error: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<XrdCl::Pipeline>::size_type’ {aka ‘long unsigned int’} [-Werror=sign-compare]
         for( int i = 0; i < pipelines.size(); i++ )
                         ~~^~~~~~~~~~~~~~~~~~
/build/xrootd/src/xrootd-4.9.0/src/XrdCl/XrdClParallelOperation.hh:95:17: error: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<XrdCl::Pipeline>::size_type’ {aka ‘long unsigned int’} [-Werror=sign-compare]
           if( i != pipelines.size() - 1 )
               ~~^~~~~~~~~~~~~~~~~~~~~~~

int i should be size_t i - no reason not to be.
Also, change the if- statement to use +. I'll attach a pull request in a bit.