Avoid fetching one past the end of translate()'s "to" parameter.

This is usually harmless, but if you were very unlucky it could
provoke a segfault due to the "to" string being right up against
the end of memory.  Found via valgrind testing (so we might've
found it earlier, except that our regression tests lacked any
exercise of translate()'s deletion feature).

Fix by switching the order of the test-for-end-of-string and
advance-pointer steps.  While here, compute "to_ptr + tolen"
just once.  (Smarter compilers might figure that out for
themselves, but let's just make sure.)

Report and fix by Daniil Anisimov, in bug #17816.

Discussion: https://postgr.es/m/17816-70f3d2764e88a108@postgresql.org

2 files changed, 7 insertions, 0 deletions

diff --git a/src/test/regress/expected/strings.out b/src/test/regress/expected/strings.out
index 6fbaf152cd1..213b236a145 100644
--- a/src/test/regress/expected/strings.out
+++ b/src/test/regress/expected/strings.out
@@ -1771,6 +1771,12 @@ SELECT translate('12345', '14', 'ax');
  a23x5
 (1 row)
 
+SELECT translate('12345', '134', 'a');
+ translate 
+-----------
+ a25
+(1 row)
+
 SELECT ascii('x');
  ascii 
 -------
diff --git a/src/test/regress/sql/strings.sql b/src/test/regress/sql/strings.sql
index 8eb6bf30af5..7cb31859e4b 100644
--- a/src/test/regress/sql/strings.sql
+++ b/src/test/regress/sql/strings.sql
@@ -608,6 +608,7 @@ SELECT ltrim('zzzytrim', 'xyz');
 
 SELECT translate('', '14', 'ax');
 SELECT translate('12345', '14', 'ax');
+SELECT translate('12345', '134', 'a');
 
 SELECT ascii('x');
 SELECT ascii('');