Null pointer dereference in void_expr_error

[clayton-shopify]

The following input demonstrates a crash:

x{|(superclass)

It appears the problem began in 762f682.

ASAN report:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==86779==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0001058d81f6 bp 0x7ffeea65f180 sp 0x7ffeea65f0e0 T0)
==86779==The signal is caused by a READ memory access.
==86779==Hint: address points to the zero page.
    #0 0x1058d81f5 in void_expr_error (mruby:x86_64+0x10033f1f5)
    #1 0x1058d6ba0 in new_masgn (mruby:x86_64+0x10033dba0)
    #2 0x1058c59cc in yyparse (mruby:x86_64+0x10032c9cc)
    #3 0x1058a42bf in mrb_parser_parse (mruby:x86_64+0x10030b2bf)
    #4 0x1058d17e1 in mrb_parse_file (mruby:x86_64+0x1003387e1)
    #5 0x1058d4046 in mrb_load_file_cxt (mruby:x86_64+0x10033b046)
    #6 0x10559ba06 in main (mruby:x86_64+0x100002a06)
    #7 0x7fff761fa08c in start (libdyld.dylib:x86_64+0x1708c)

==86779==Register values:
rax = 0x0000000000000000  rbx = 0x00007ffeea65fb80  rcx = 0x00006280000044c8  rdx = 0x0000000000000000
rdi = 0x0000628000004120  rsi = 0x0000100000000000  rbp = 0x00007ffeea65f180  rsp = 0x00007ffeea65f0e0
 r8 = 0x0000000000000142   r9 = 0x0000628000004100  r10 = 0x0000000000000000  r11 = 0x0000000000000690
r12 = 0x00007ffeea65f200  r13 = 0x00007ffeea65f2a0  r14 = 0x000000010594bfe4  r15 = 0x00007ffeea65f1e0
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mruby:x86_64+0x10033f1f5) in void_expr_error
==86779==ABORTING
Abort trap: 6

This issue was reported by https://hackerone.com/hexodus