check that cumulus port names cannot map to negative line numbers

cmd/metal-api/internal/metal/switch.go

@@ -312,12 +312,18 @@ func cumulusPortNameToLine(port string) (int, error) {
 		if err != nil {
 			return 0, fmt.Errorf("unable to convert port name to line number: %w", err)
 		}
+		if count <= 0 {
+			return 0, fmt.Errorf("invalid port name would map to negative number")
+		}
 		line = (count - 1) * 4
 	} else {
 		count, err := strconv.Atoi(countString)
 		if err != nil {
 			return 0, fmt.Errorf("unable to convert port name to line number: %w", err)
 		}
+		if count <= 0 {
+			return 0, fmt.Errorf("invalid port name would map to negative number")
+		}
 
 		index, err := strconv.Atoi(indexString)
 		if err != nil {

cmd/metal-api/internal/metal/switch_test.go

@@ -676,10 +676,22 @@ func Test_cumulusPortNameToLine(t *testing.T) {
 		},
 		{
 			name:    "cannot convert second number",
-			port:    "swp0s_0",
+			port:    "swp1s_0",
 			want:    0,
 			wantErr: fmt.Errorf("unable to convert port name to line number: %w", parseIntError2),
 		},
+		{
+			name:    "cannot convert swp0 because that would result in a negative line number",
+			port:    "swp0",
+			want:    0,
+			wantErr: fmt.Errorf("invalid port name would map to negative number"),
+		},
+		{
+			name:    "cannot convert swp0s1 because that would result in a negative line number",
+			port:    "swp0s1",
+			want:    0,
+			wantErr: fmt.Errorf("invalid port name would map to negative number"),
+		},
 		{
 			name:    "convert line without breakout",
 			port:    "swp4",