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/muscle-tab.c

@@ -128,6 +128,8 @@ muscle_init (void)
 
   muscle_table = hash_initialize (HT_INITIAL_CAPACITY, NULL, hash_muscle,
                                   hash_compare_muscles, muscle_entry_free);
+  if (!muscle_table)
+    xalloc_die ();
 
   /* Version and input file.  */
   MUSCLE_INSERT_STRING ("version", VERSION);