函数错误:未分配正在分配的指针

I was trying to use realloc function in implementing a string combination function. This function is going to merge two strings, leftPiece and rightPiece, the tail of left piece and head of right piece has overlapped characters with number num_overlap. Because the input strings are perfectly sized, I have to reallocate the string so that I have enough space to concatenate the two strings. My function is as follows:I was trying to use realloc function in impleme