mdocml: Don't deref NULL if the only child of the first .Sh is an empty

[schwarze@mdocml.bsd.lv]

Log Message:
-----------
Don't deref NULL if the only child of the first .Sh is an empty
in-line macro, and don't printf("%s", NULL) if the first child
of the first .Sh is a macro; again found by tb@ with afl(1).
(No, you should never use macros in any .Sh at all, please.)

Modified Files:
--------------
    mdocml:
        mdoc_validate.c

Revision Data
-------------
Index: mdoc_validate.c
===================================================================
RCS file: /home/cvs/mdocml/mdocml/mdoc_validate.c,v
retrieving revision 1.304
retrieving revision 1.305
diff -Lmdoc_validate.c -Lmdoc_validate.c -u -p -r1.304 -r1.305
--- mdoc_validate.c
+++ mdoc_validate.c
@@ -1761,8 +1761,9 @@ post_sh_authors(POST_ARGS)
 static void
 post_sh_head(POST_ARGS)
 {
-	const char	*goodsec;
-	enum roff_sec	 sec;
+	struct roff_node	*nch;
+	const char		*goodsec;
+	enum roff_sec		 sec;
 
 	/*
 	 * Process a new section.  Sections are either "named" or
@@ -1778,8 +1779,10 @@ post_sh_head(POST_ARGS)
 	if (sec != SEC_NAME && mdoc->lastnamed == SEC_NONE)
 		mandoc_vmsg(MANDOCERR_NAMESEC_FIRST, mdoc->parse,
 		    mdoc->last->line, mdoc->last->pos, "Sh %s",
-		    sec == SEC_CUSTOM ? mdoc->last->child->string :
-		    secnames[sec]);
+		    sec != SEC_CUSTOM ? secnames[sec] :
+		    (nch = mdoc->last->child) == NULL ? "" :
+		    nch->type == ROFFT_TEXT ? nch->string :
+		    mdoc_macronames[nch->tok]);
 
 	/* The SYNOPSIS gets special attention in other areas. */
 
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