check for memory exhaustion

hash_initialize returns NULL when out of memory.  Check for it, and
die cleanly instead of crashing.

Reported by 江 祖铭 (Zu-Ming Jiang).
https://lists.gnu.org/archive/html/bug-bison/2019-08/msg00015.html

* src/muscle-tab.c, src/state.c, src/symtab.c, src/uniqstr.c:
Check the value returned by hash_initialize.

src/uniqstr.c

@@ -162,6 +162,8 @@ uniqstrs_new (void)
                                     hash_uniqstr,
                                     hash_compare_uniqstr,
                                     free);
+  if (!uniqstrs_table)
+    xalloc_die ();
 }