Coverity related changes

These should fix most of the reported Coverity issues.  The remaining issues
should be the back_b etc assignments, which look like a Coverity bug, and
passing a pointer to a byte to a function that expects a pointer to one or more
bytes, which should (I believe) be fixed in one case and not the other
(next_filter) case; the latter case will probably go away as I am going to
rewrite that piece of code to avoid a spurious buffer allocation.

Signed-off-by: John Bowler <jbowler@acm.org>

ANNOUNCE

@@ -1,5 +1,5 @@
 
-Libpng 1.7.0beta66 - September 17, 2015
+Libpng 1.7.0beta66 - September 18, 2015
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -871,7 +871,7 @@ Version 1.7.0beta65 [September 16, 2015]
     (but not yet by the writer) (John Bowler).
   Implemented shared transform handling that is used throughout (John Bowler).
 
-Version 1.7.0beta66 [September 17, 2015]
+Version 1.7.0beta66 [September 18, 2015]
   Enabled the low-bit-depth gray tests that were disabled in prior versions
     of libpng because of problems that should have been fixed by the recent
     changes to libpng17. Enabling the tests revealed bugs in those changes
@@ -881,6 +881,7 @@ Version 1.7.0beta66 [September 17, 2015]
     be copied, also the copies are improved to copy the transform args too;
     not required at present but it may prevent a bug being introduced in
     the future.
+  Fixed some new Coverity defects that were introduced in 1.7.0beta65.
 
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