My question is : it is secure to use a simple "if" to check the licence ?
Yes.
Can anyone add a simple "jmp" in assembly language to "jump" this simple if ?
Yes, everything can be cracked. See: "What is hardware-locked licensing" where we cover this in detail.
No licensing can prevent cracking, but that's not the point of licensing. The point of licensing is to increase revenue by preventing casual piracy (sharing keys among end-users and corporate customers).