Revert "Merge pull request #3126 from jbreeden/backtrace_irep_null_check"

This reverts commit bf7719fe, reversing
changes made to 4f4fa0ad.

We should get backtrace while irep is alive.

src/backtrace.c

@@ -112,8 +112,6 @@ each_backtrace(mrb_state *mrb, mrb_int ciidx, mrb_code *pc0, each_backtrace_func
     mrb_irep *irep;
     mrb_code *pc;
 
-    loc.lineno = -1;
-
     ci = &mrb->c->cibase[i];
 
     if (!ci->proc) continue;
@@ -130,11 +128,8 @@ each_backtrace(mrb_state *mrb, mrb_int ciidx, mrb_code *pc0, each_backtrace_func
     else {
       pc = pc0;
     }
-    
-    if (irep) {
     loc.filename = mrb_debug_get_filename(irep, (uint32_t)(pc - irep->iseq));
     loc.lineno = mrb_debug_get_line(irep, (uint32_t)(pc - irep->iseq));
-    }
 
     if (loc.lineno == -1) continue;