Can my current company stop me in joining other company?

Currently I am working in an IT services provider company, say ABC. I worked on some projects for their client, say HAL. I have got an job offer from a company SLB and I have accepted their offer. Now I came to know, not from official sources, that this SLB or its sister company is client of my current company ABC. And as per my appointment letter from current company, I can not join their client for 12 months from the date of relieving. The exact clause mentioned in letter is as follows: "You are not eligible to join any client/Customer/ sister concerns and/or subsidiaries of Company without approval of Company for a period of 12 months from your date of relieving." My worry is that if can my current company can sue me for joining SLB? Some points that might help: 1. I never worked on any projects involving SLB as a client. 2. I did not had any information that SLB is client of ABC. No one from company gave me this information. 3. I got this offer from SLB by applying independently. 4. No where in the appointment letter from ABC, they have defined "Client" word. I shall really appreciate any advice in this matter.