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.

THANKS

@@ -199,6 +199,7 @@ Wwp                       subscript@free.fr
 xolodho                   xolodho@gmail.com
 Zack Weinberg             zack@codesourcery.com
 長田偉伸                   cbh34680@iret.co.jp
+江 祖铭                    jjzuming@outlook.com
 
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