lalr: fix segmentation violation

The "includes" relation [DeRemer 1982] is between gotos, so of course,
for a given goto, there cannot be more that ngotos (number of gotos)
images.  But we manipulate the set of images of a goto as a list,
without checking that an image was not already introduced.  So we can
"register" way more images than ngotos, leading to a crash (heap
buffer overflow).

Reported by wcventure.
http://lists.gnu.org/archive/html/bug-bison/2019-03/msg00007.html

For the records, this bug is present in the first committed version of
Bison.

* src/lalr.c (build_relations): Don't insert the same goto several
times.
* tests/sets.at (Build Relations): New.

THANKS

@@ -176,6 +176,7 @@ Tommy Nordgren            tommy.nordgren@chello.se
 Troy A. Johnson           troyj@ecn.purdue.edu
 Tys Lefering              gccbison@gmail.com
 Valentin Tolmer           nitnelave1@gmail.com
+wcventure                 wcventure@126.com
 Victor Khomenko           victor.khomenko@newcastle.ac.uk
 Victor Zverovich          victor.zverovich@gmail.com
 Vin Shelton               acs@alumni.princeton.edu