ccanlint: avoid segfault when module_builds' linking fails

In the case that the objects built but linking failed, module_builds.c
called score_file_error with a NULL ccan_file object and 0 for line
number.

score_file_error assumed that the ccan_file object it is passed was
not-NULL when appending file errors to the score's aggregate error
string.  It attempted to dereference it to get "fullname".

score_error was factored out from score_file_error.  It takes a
"source" parameter, which is the file's full name (and possibly line
number) in the score_file_error case, and the ccan module name in the
case of link failure.

tools/ccanlint/ccanlint.h

@@ -152,6 +152,10 @@ char **per_file_options(const struct ccanlint *test, struct ccan_file *f);
 void score_file_error(struct score *, struct ccan_file *f, unsigned line,
 		      const char *errorfmt, ...);
 
+/* Append message to the score->error */
+void score_error(struct score *score, const char * source,
+		 const char *errorfmt, ...);
+
 /* Start a command in the background. */
 void run_command_async(const void *ctx, unsigned int time_ms,
 		       const char *fmt, ...);

tools/ccanlint/file_analysis.c

@@ -360,15 +360,9 @@ enum line_compiled get_ccan_line_pp(struct pp_conditions *cond,
 	return ret;
 }
 
-void score_file_error(struct score *score, struct ccan_file *f, unsigned line,
-		      const char *errorfmt, ...)
+static void score_error_vfmt(struct score *score, const char *source,
+			     const char *errorfmt, va_list ap)
 {
-	va_list ap;
-
-	struct file_error *fe = tal(score, struct file_error);
-	fe->file = f;
-	fe->line = line;
-	list_add_tail(&score->per_file_errors, &fe->list);
 
 	if (!score->error)
 		score->error = tal_strdup(score, "");
@@ -384,17 +378,45 @@ void score_file_error(struct score *score, struct ccan_file *f, unsigned line,
 		return;
 	}
 
+	tal_append_fmt(&score->error, "%s:", source);
+	tal_append_vfmt(&score->error, errorfmt, ap);
+	score->error = tal_strcat(score, take(score->error), "\n");
+}
+
+
+
+void score_error(struct score *score, const char *source,
+		 const char *errorfmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, errorfmt);
+	score_error_vfmt(score, source, errorfmt, ap);
+	va_end(ap);
+}
+
+void score_file_error(struct score *score, struct ccan_file *f, unsigned line,
+		      const char *errorfmt, ...)
+{
+	va_list ap;
+	char *source;
+
+	struct file_error *fe = tal(score, struct file_error);
+	fe->file = f;
+	fe->line = line;
+	list_add_tail(&score->per_file_errors, &fe->list);
+
 	if (line)
-		tal_append_fmt(&score->error, "%s:%u:", f->fullname, line);
+		source = tal_fmt(score, "%s:%u", f->fullname, line);
 	else
-		tal_append_fmt(&score->error, "%s:", f->fullname);
+		source = tal_fmt(score, "%s", f->fullname);
 
 	va_start(ap, errorfmt);
-	tal_append_vfmt(&score->error, errorfmt, ap);
+	score_error_vfmt(score, source, errorfmt, ap);
 	va_end(ap);
-	score->error = tal_strcat(score, take(score->error),"\n");
 }
 
+
 char *get_or_compile_info(const void *ctx, const char *dir)
 {
 	struct manifest *m = get_manifest(NULL, dir);

tools/ccanlint/tests/module_builds.c

@@ -57,7 +57,7 @@ static void do_build(struct manifest *m,
 	m->compiled[COMPILE_NORMAL]
 		= build_module(m, COMPILE_NORMAL, &errstr);
 	if (!m->compiled[COMPILE_NORMAL]) {
-		score_file_error(score, NULL, 0, "%s", errstr);
+		score_error(score, m->modname,"%s", errstr);
 		return;
 	}