fix #18136 tests/proc/t17157.nim now gives SIGSEGV instead of error (#20555)

compiler/semtypes.nim

@@ -1257,6 +1257,7 @@ proc newProcType(c: PContext; info: TLineInfo; prev: PType = nil): PType =
   result.n.add newNodeI(nkEffectList, info)
 
 proc isMagic(sym: PSym): bool =
+  if sym.ast == nil: return false
   let nPragmas = sym.ast[pragmasPos]
   return hasPragma(nPragmas, wMagic)
 

tests/proc/t17157.nim

@@ -1,6 +1,5 @@
 discard """
   errormsg: "'untyped' is only allowed in templates and macros or magic procs"
-  disabled: true
 """
 
 template something(op: proc (v: untyped): void): void =