When a literal string is used to define two different tokens,

`bison -v' segfaults.
Reported by Piotr Gackiewicz, and fixed by Neil Booth.
* tests/regression.m4: New file.
Include the core of the sample provided by Piotr Gackiewicz.
* src/reader.c (parse_token_decl): Diagnose bad cases, and proceed
properly.

tests/regression.m4

@@ -0,0 +1,26 @@
+#                                                        -*- Autoconf -*-
+
+cat <<EOF
+
+Regression tests.
+
+EOF
+
+AT_SETUP(Duplicate string)
+
+AT_DATA([duplicate.y],
+[[/* `Bison -v' used to dump core when two tokens are defined with the same
+   string, as LE and GE below. */
+
+%token NUM
+%token LE "<="
+%token GE "<="
+
+%%
+exp: '(' exp ')' | NUM ;
+%%
+]])
+
+AT_CHECK([bison -v duplicate.y -o duplicate.c], 0, ignore, ignore)
+
+AT_CLEANUP(duplicate.*)