ielr: fix crash on memory management

Reported by Dwight Guth.
https://lists.gnu.org/r/bug-bison/2020-06/msg00037.html

* src/AnnotationList.c (AnnotationList__computePredecessorAnnotations):
Beware that SBITSET__FOR_EACH nests _two_ for-loops, so "break" does
not actually break out of it.
That was the only occurrence in the code.
* src/Sbitset.h (SBITSET__FOR_EACH): Warn passersby.

NEWS

@@ -44,6 +44,10 @@ GNU Bison NEWS
   string aliases semantically equivalent but syntactically different (e.g.,
   "A", "\x41", "\101") are considered to be different.
 
+*** Crash when generating IELR
+
+  An old, well hidden, bug in the generation of IELR parsers was fixed.
+
 ** New features
 
 *** File prefix mapping