[process][linux] Fix NewProcess() on Linux

Related to #704.
Don't break previous API where a Process is always returned, fix undefined variable p.

process/process_linux.go

@@ -69,10 +69,8 @@ func (m MemoryMapsStat) String() string {
 // to get more information about the process. An error will be returned
 // if the process does not exist.
 func NewProcess(pid int32) (*Process, error) {
-	if _, err := os.Stat(common.HostProc(strconv.Itoa(int(p.Pid)))); err != nil {
-		return nil, err
-	}
-	return &Process{Pid: int32(pid)}, nil
+	_, err := os.Stat(common.HostProc(strconv.Itoa(int(pid))))
+	return &Process{Pid: pid}, err
 }
 
 // Ppid returns Parent Process ID of the process.