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/symtab.c

@@ -778,11 +778,15 @@ symbols_new (void)
                                   hash_symbol_hasher,
                                   hash_symbol_comparator,
                                   symbol_free);
+  if (!symbol_table)
+    xalloc_die ();
   semantic_type_table = hash_initialize (HT_INITIAL_CAPACITY,
                                          NULL,
                                          hash_semantic_type_hasher,
                                          hash_semantic_type_comparator,
                                          free);
+  if (!semantic_type_table)
+    xalloc_die ();
 }