fix issue 84

[achille-roussel]

Fixes #84

This PR modifies the functions in the ascii package to prevent pointers being advanced beyond the end of the memory areas they started in.

Technically, we were never dereferencing pointers with addresses beyond the allocated areas, but Go makes it invalid to simply have pointers that address unallocated memory. To my understanding, this is done to prevent exposing out-of-bounds pointers to the GC, which could mistakenly create references to objects that shouldn't have any, or potentially cause runtime errors if the GC finds pointers that aren't mapping to any allocated areas.

I believe that an alternative solution could have been to use //go:nosplit on the functions, which should prevent these "out-of-bounds" pointers from being seen by the GC, tho I haven't found enough literature on the subject to feel confident enough in that assumption.

Thanks @aaron42net for reporting the issue!

[chriso]

Can we incorporate -race into our test commands so we catch these issues early?

Perhaps something like this:

diff --git i/Makefile w/Makefile
index 09c0db2..f70b7a0 100644
--- i/Makefile
+++ w/Makefile
@@ -11,13 +11,8 @@ go-fuzz-corpus := ${GOPATH}/src/github.com/dvyukov/go-fuzz-corpus
 go-fuzz-dep := ${GOPATH}/src/github.com/dvyukov/go-fuzz/go-fuzz-dep
 
 test:
-   go test -v -cover ./ascii
-   go test -v -cover ./json
-   go test -v -cover ./proto
-   go test -v -cover -tags go1.15 ./json
-   go test -v -cover ./iso8601
-   go run ./json/bugs/issue11/main.go
-   go run ./json/bugs/issue18/main.go
+   go test -v -race -cover ./...
+   go test -v -race -cover -tags go1.17 ./json
 
 $(benchstat):
    GO111MODULE=off go get -u golang.org/x/perf/cmd/benchstat

If we use the same main_test.go pattern in the issue11/issue18 dirs then they'll be run automatically by go test ./...

[achille-roussel]

@chriso I brought your suggestions in, but kept each test separated so we can run them in parallel with make -j (running with -race takes way more time).