mismatching closing parenthesis causes segfault

[ennorehling]

ceval will crash when given an expression like ")", "1)", or "pi)":

(gdb) r ')'
Starting program: /home/enno/ceval/example ')'

Program received signal SIGSEGV, Segmentation fault.
0x0000555555557e04 in ceval_insert_node (current=0x7fffffffddd0, item=..., isRightAssoc=1) at ./core/parser.h:41
41              parent_of_openpar -> right = current -> right;