re2: Remove comparisons of this with NULL.

Remove comparisons of this with NULL.

Calling functions on objects that are NULL is undefined behavior.
Hence, new versions of clang assume that `this` is never NULL,
warn on comparisons of `this` with NULL, and optimize the comparison away.

There were three comparisons of `this` with NULL in re2:

1. CharClass::Delete(). There are only two callers of this method, and
   as far as I can tell `this` can't be NULL there, so just delete that check.
2. Prefilter::DebugString(). This too has two callers: Prefilter::Info::ToString(),
   which already checks for NULL before calling, and DebugString() itself.
   Since several places check Prefilters for NULL, add explicit checks before
   calling here.
3. Prefilter::Info::ToString(). This has three callers. In 2 cases this can't be
   NULL. In the third case it could conceivably be NULL if op() is
   kRegexpConcat and nchild_args is 0, so add an explicit check here.
   
Fixes https://code.google.com/p/re2/issues/detail?id=115

[thakis, 2014-06-13 23:36:52]

`make test` is happy. Someone of the tests I added might not be necessary.

[thakis, 2014-06-23 15:57:05]

Ping?

[rsc, 2014-06-26 15:03:11]

I guess this is correct but it's really annoying.

[rsc, 2014-06-26 15:03:38]

LGTM

[rsc, 2014-06-26 15:20:37]

*** Submitted as https://code.google.com/p/re2/source/detail?r=b92ce81f1e25 ***

Remove comparisons of this with NULL.

Calling functions on objects that are NULL is undefined behavior.
Hence, new versions of clang assume that `this` is never NULL,
warn on comparisons of `this` with NULL, and optimize the comparison away.

There were three comparisons of `this` with NULL in re2:

1. CharClass::Delete(). There are only two callers of this method, and
   as far as I can tell `this` can't be NULL there, so just delete that check.
2. Prefilter::DebugString(). This too has two callers:
Prefilter::Info::ToString(),
   which already checks for NULL before calling, and DebugString() itself.
   Since several places check Prefilters for NULL, add explicit checks before
   calling here.
3. Prefilter::Info::ToString(). This has three callers. In 2 cases this can't be
   NULL. In the third case it could conceivably be NULL if op() is
   kRegexpConcat and nchild_args is 0, so add an explicit check here.

Fixes https://code.google.com/p/re2/issues/detail?id=115

LGTM=rsc
R=rsc
CC=re2-dev
https://codereview.appspot.com/107100043

Committer: Russ Cox <rsc@swtch.com>