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            [email protected]
 Troy A. Johnson           [email protected]
 Tys Lefering              [email protected]
 Valentin Tolmer           [email protected]
+wcventure                 [email protected]
 Victor Khomenko           [email protected]
 Victor Zverovich          [email protected]
 Vin Shelton               [email protected]