Binary add segfaults when at least 1 arg is non-numeric, and both are non-pointer types

[StrongerXi]

Problem

The following program segfaults in new_add function from parse.c

void foo() { }

void bar() {
  1 + foo();
}

Proposal

Check for the case when both args are non-pointer types; check NULL explicitly to improve readability.

Note the following program still compiles with or without this patch:

void foo() { }

void bar() {
  int *a;
  a + foo();
}

It could be fixed easily, but like mentioned in #41, a separate "typecheck" phase would probably be a better solution IMHO.