Fix stack-overflow in Binary_Expression

[xzyfer]

  | ==1==ERROR: AddressSanitizer: stack-overflow on address 0x7ffd4469eff8 (pc 0x000000b87f3e bp 0x7ffd4469f030 sp 0x7ffd4469f000 T0)
-- | --
  | #0 0xb87f3e in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:256:14
  | #1 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #2 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #3 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #4 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #5 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #6 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #7 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #8 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #9 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #10 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #11 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #12 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #13 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #14 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #15 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #16 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13
  | #17 0xb880ac in Sass::Binary_Expression::set_delayed(bool) libsass/src/ast_values.cpp:257:13