style: prefer snprintf to sprintf

* src/symtab.c (dummy_symbol_get): There's no need for the buffer to
be so big and static.
Use snprintf for safety.

bootstrap.conf

@@ -17,7 +17,7 @@
 
 # gnulib modules used by this package.
 gnulib_modules='
-  argmatch array-list assert
+  argmatch array-list assert assure
   bitsetv
   calloc-posix close closeout config-h c-strcase
   configmake