micro fix for system.paramstr()
rtl/linux/system.pp
(and for other Unixes too?)
```
function paramstr(l: longint) : string;
begin
{ stricly conforming POSIX applications }
{ have the executing filename as argv[0] }
if l=0 then
begin
if execpathstr='' then
SysInitExecPath;
paramstr := execpathstr;
end
else if (l < argc) then
paramstr:=strpas(argv[l])
else
paramstr:='';
end;
```
Here we miss the check "if (l>0)" to avoid crash on negative argument.
issue