CWE-170 Improper Null Termination
CWE-119 Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer
Notice how in this case, CWE-119 would be resultant, and CWE-682 would be primary. Due to an incorrect calculation (CWE-682), proper null termination would not occur (CWE-170), and ultimately the program might read or write memory outside of a buffer (CWE-119). The chaining relationship of these three weaknesses could introduce a vulnerability.
The question then becomes how to classify CWE-170, the middle step of this chain. During this step, the associated behavior does not properly maintain the structure of data (i.e. the null termination), which is relied on by the code. So, this falls under CWE-707 (Failure to Enforce that Messages or Data are Well-Formed).