MINOR: kern_procargs more robut on darwin

Under some circonstance the code can fail, the root cause is not clear
at the moment, but we should still make the code more robust when
retrieving information for a specific process.

The problem is the `bytes.SplitN` can return an array of one element
making the process panic when we access the other element.

This commit make the code a bit more robust and check to make sure we
successfully retrieved 2 elements instead of one and return an error
if it failed.

Reference: elastic/beats#5337

sigar_darwin.go

@@ -420,7 +420,12 @@ func kern_procargs(pid int,
 			return fmt.Errorf("Error reading args: %v", err)
 		}
 		pair := bytes.SplitN(chop(line), delim, 2)
-		env(string(pair[0]), string(pair[1]))
+
+		if len(pair) == 2 {
+			env(string(pair[0]), string(pair[1]))
+		} else {
+			return fmt.Errorf("Error reading process information for PID: %d", pid)
+		}
 	}
 
 	return nil