Perl_deb_stack_all() - handle empty CX stack

This function handles perl -Dsv, producing output like

STACK 0: MAIN
  CX 0: BLOCK  =>
  CX 1: SUB    => UNDEF  PV("main"\0)
  retop=leave
STACK 1: MAGIC
  CX 0: SUB    => IV(1)

When a CX stack had zero contexts pushed (like can sometimes happen
when something has just done a PUSHSTACKi() and no op has pushed a BLOCK
or SUB or whatever yet), then the code for determining where the next
markstack pointer is (by peeking ahead into the first CX of the next SI)
was accessing random garbage at cx[0]. This commit fixes that.

Author: iabyn

deb.c

@@ -320,7 +320,10 @@ Perl_deb_stack_all(pTHX)
                     }
                     if (CxTYPE(&(si_n->si_cxstack[i])) == CXt_SUBST)
                         continue;
-                    cx_n = &(si_n->si_cxstack[i]);
+                    if (si_n->si_cxix >= 0)
+                        cx_n = &(si_n->si_cxstack[i]);
+                    else
+                        cx_n = NULL;
                     break;
                 }