bug#19208: replace-match unhelpful error message

[Paul Eggert]

> No, that's just another sanity check -- num_regs is the max allowed
> number of sub-matches.  (I've now added some comments to clarify.)

Unfortunately that patch messes up the sanity check, as the patched code allows 
'sub' to be negative, or to be equal to search_regs.num_regs, and in either case 
this results in a bad pointer.

> Yoda conditionals and a !...

Actually those were Leibniz conditionals, which are comparisons involving "<" or 
"<=". The idea is that the conditionals' textual order reflects numeric order. 
This is a common style in math when doing range checking, e.g., "0 <= i < n". 
Yoda conditionals are expressions like "0 != x" which I agree are confusing.

I learned Leibniz conditionals from the late Val Schorre, who was *really* good 
at programming and programming style: Knuth credits Schorre with the invention 
of goto-less programming in the early 1960s. When I redid your patch as a 
Leibniz conditional, I instantly spotted the bug that it introduced. You might 
give Leibniz conditionals a try, as they help make code more readable and 
reliable when checking for range errors or overflow.

While in the neighborhood I spotted some other rare possibilities for bad 
behavior due to out-of-range indexes. I fixed the bugs I found by installing the 
attached.

0001-Fix-rare-undefined-behaviors-in-replace-match.patch
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