PR# 13838 Missing critical section in `expop_helper' in except.c at chunk boundary

Based on code inspection only, it appears that there is a missing
critical section in `expop_helper' in except.c, in the unusual
case where we are at a chunk boundary.  There are several macros in
eif_macros.h that call `expop' (apparently without blocking signals)
and `expop' calls `expop_helper'.  In `expop_helper' we have

                        /* Unusual case: top pointed to the arena of next chunk */

                s= stk->st_cur = stk->st_cur->sk_prev;          /* Go one chunk back */

                CHECK("Not underflow", s);
                top = stk->st_end = s->sk_end;                          /* Set new end */
                stk->st_top = --top;                                            /* New top */

If signals aren't blocked and we get a signal where the CHECK is, then
stk->st_cur will have been updated to point to previous chunk but 
stk->st_end stk->st_top won't have been updated, resulting in a messed
up stack.  The SIGBLOCK/SIGRESUME should also enclose the entire
"else" clause, including the stack_truncate call, to avoid possible
memory leaks.

I'm guessing that the similar code for st_truncate in garcol.c doesn't
have a problem because all callers block signals.  Perhaps documentation
for run-time routines needs to be extended to include information on
whether signals must be blocked by the caller.

Looks like `c_npop' in debug.c has the same problem, in the case where
top == arena.  Presumably, this code was copied from garcol.c and adapted.