parseaut: fix handling of [ outside HOA

Fixes #509.

* spot/parseaut/scanaut.ll: Reset ->str whenever a [ is read,
so that we do not attempt to clear ->str while reading garbage.
* NEWS: Mention the bug.
* tests/core/parseaut.test: Test it.

NEWS

@@ -124,6 +124,10 @@ New in spot 2.10.6.dev  (not yet released)
     hide true state could produce some incorrect GraphViz output if
     the automaton as a true state as part of a universal group.
 
+  - Due to an optimization introduces in 2.10 to parse HOA label more
+    efficiently, the automaton parser could crash when parsing random
+    input (not HOA) containing '[' (issue #509).
+
 New in spot 2.10.6  (2022-05-18)
 
   Bugs fixed:

spot/parseaut/scanaut.ll

@@ -454,6 +454,11 @@ identifier  [[:alpha:]_][[:alnum:]_.-]*
 			}
 }
 
+"["                     {
+                          yylval->str = nullptr;
+                          return *yytext;
+                        }
+
 .			return *yytext;
 
 %{

tests/core/parseaut.test

@@ -2961,3 +2961,13 @@ EOF
 # At some point, this crashed with
 # input buffer overflow, can't enlarge buffer because scanner uses REJECT
 run 0 autfilt -q bigaut
+
+
+# This issued to segfault, because the parser assumed a '[' token was
+# always attached to a string, while that was only true in HOA mode.
+cat >input  <<EOF
+"foo" [t] --END--
+EOF
+autfilt -q input && exit 1
+test $? -eq 2
+: