ivlpp: Fix segfault in macro_start_args

macro_start_args truncates def_buf, but does not check to ensure that
someone has allocated def_buf first. This will cause a null pointer
dereference if the first access to def_buf while parsing a file is a
macro invocation. Fix this by avoiding truncating def_buf if it is NULL,
as it is effectively already truncated.

Fixes: 680196953 ("Add support for text macros with arguments.")
Signed-off-by: Sean Anderson <seanga2@gmail.com>

ivlpp/lexor.lex

@@ -1485,8 +1485,10 @@ static void macro_start_args(void)
      * entry for arg 0. This will be used by macro_finish_arg() to
      * calculate the buffer location for arg 1.
      */
-    def_buf_free = def_buf_size - 1;
-    def_buf[0] = 0;
+    if (def_buf) {
+      def_buf_free = def_buf_size - 1;
+      def_buf[0] = 0;
+    }
     def_argo[0] = 0;
     def_argl[0] = 0;
     def_argc = 1;

ivtest/ivltests/macro_args.v

@@ -0,0 +1,8 @@
+`ifndef FOO
+`define FOO
+`define BAR(x)
+`endif
+
+module macro_args();
+	macro_args_sub sub();
+endmodule

ivtest/ivltests/macro_args_sub.v

@@ -0,0 +1,3 @@
+module macro_args_sub();
+	`BAR(0)
+endmodule

ivtest/regress-vlg.list

@@ -628,6 +628,7 @@ lh_varindx5		normal			ivltests
 localparam_type		normal			ivltests gold=parameter_type.gold
 long_div		normal			ivltests gold=long_div.gold
 macro2			normal			ivltests
+macro_args		CO,-yivltests			ivltests
 macro_redefinition	normal,-Wmacro-redefinition	ivltests gold=macro_redefinition.gold
 macro_replacement	normal,-Wmacro-replacement	ivltests gold=macro_replacement.gold
 macsub			normal			ivltests