Update symbolic_path.cc

The buffer size (4096) is too small. When I test the program grep, CREST-z3 breaks down soon. I think the MAX_LINE_BUF should be set to 4000000*sizeof(branch_id_t), since the maximum size of branches is set to 4000000 at line 26. However, the number4000000 is too large for many systems to allocate such size memory!