Fix checks for null HANDLE in Windows-only code

clang-cl failed with "error: unordered comparison between pointer and zero
('HANDLE' (aka 'void *') and 'int')" in these two places introduced with
f219283 "Sub-task CORE-4463: Windows
implementation for CORE-4462 (Make it possible to restore compressed .nbk files
without explicitly decompressing them)" and
c2cfa78 "Prevent child process hung if it
writes too much data to the pipe and overflow the pipe buffer".

src/utilities/nbackup/nbackup.cpp

@@ -385,7 +385,7 @@ FB_SIZE_T NBackup::read_file(FILE_HANDLE &file, void *buffer, FB_SIZE_T bufsize)
 #ifdef WIN_NT
 		// Read child's stderr often to prevent child process hung if it writes
 		// too much data to the pipe and overflow the pipe buffer.
-		const bool checkChild = (childStdErr > 0 && file == backup);
+		const bool checkChild = (childStdErr != 0 && file == backup);
 		if (checkChild)
 			print_child_stderr();
 
@@ -790,7 +790,7 @@ void NBackup::close_backup()
 		return;
 #ifdef WIN_NT
 	CloseHandle(backup);
-	if (childId > 0)
+	if (childId != 0)
 	{
 		const bool killed = (WaitForSingleObject(childId, 5000) != WAIT_OBJECT_0);
 		if (killed)