Avoid crash with "NSString" literals

Evaluating an Objective-C "NSString" literal will cause gdb to crash.
This patch fixes the crash.

I think the result here still isn't correct -- I see a warning from
the runtime ("autorelease called without pool for object") with the
new code.

However, not crashing is an improvement on its own.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20501

gdb/c-exp.y

@@ -223,7 +223,7 @@ static void c_print_token (FILE *file, int type, YYSTYPE value);
    nonterminal "name", which matches either NAME or TYPENAME.  */
 
 %token <tsval> STRING
-%token <sval> NSSTRING		/* ObjC Foundation "NSString" literal */
+%token <tsval> NSSTRING		/* ObjC Foundation "NSString" literal */
 %token SELECTOR			/* ObjC "@selector" pseudo-operator   */
 %token <tsval> CHAR
 %token <ssym> NAME /* BLOCKNAME defined below to give it higher precedence. */
@@ -1030,12 +1030,12 @@ exp	:	string_exp
 			}
 	;
 
-exp     :	NSSTRING	/* ObjC NextStep NSString constant
-				 * of the form '@' '"' string '"'.
-				 */
+exp     :	NSSTRING
 			{
+			  /* ObjC NextStep NSString constant of the
+			     form '@' '"' string '"'.  */
 			  pstate->push_new<objc_nsstring_operation>
-			    (copy_name ($1));
+			    (std::string ($1.ptr, $1.length));
 			}
 	;
 

gdb/objc-lang.c

@@ -169,7 +169,8 @@ objc_nsstring_operation::evaluate (struct type *expect_type,
   struct type *type;
 
   if (!target_has_execution ())
-    return 0;		/* Can't call into inferior to create NSString.  */
+    error (_("evaluation of this expression "
+	     "requires the target program to be active"));
 
   stringValue[2] = value_string (str.c_str (), str.size () + 1, char_type);
   stringValue[2] = value_coerce_array (stringValue[2]);

gdb/testsuite/gdb.objc/print.exp

@@ -62,6 +62,10 @@ clean_restart
 if { [set_lang_objc] } {
     test_float_accepted
     test_float_rejected
+
+    gdb_test {print @"hi"} \
+	"evaluation of this expression requires the target program to be active" \
+	"simple nsstring"
 } else {
     warning "Objective-c print tests suppressed"
 }