startup: do not store an error message in the record of another buffer

Set the 'format' of a file only when it has been fully read in,
so that this field can be used to indicate that any later error
message cannot be meant for this buffer.

This fixes https://savannah.gnu.org/bugs/?60269.

Bug existed since commit 6bf52dcc from yesterday.

src/definitions.h

@@ -246,7 +246,7 @@
 
 /* Enumeration types. */
 typedef enum {
-	NIX_FILE, DOS_FILE, MAC_FILE
+	UNSPECIFIED, NIX_FILE, DOS_FILE, MAC_FILE
 } format_type;
 
 typedef enum {

src/files.c

@@ -83,7 +83,7 @@ void make_new_buffer(void)
 	openfile->mark = NULL;
 	openfile->softmark = FALSE;
 
-	openfile->fmt = NIX_FILE;
+	openfile->fmt = UNSPECIFIED;
 
 	openfile->undotop = NULL;
 	openfile->current_undo = NULL;
@@ -821,9 +821,11 @@ void read_file(FILE *f, int fd, const char *filename, bool undoable)
 						num_lines), num_lines);
 	}
 #endif
-	else
+	else {
+		openfile->fmt = NIX_FILE;
 		statusline(HUSH, P_("Read %zu line", "Read %zu lines",
 						num_lines), num_lines);
+	}
 
 	/* If we inserted less than a screenful, don't center the cursor. */
 	if (undoable && less_than_a_screenful(was_lineno, was_leftedge))

src/winio.c

@@ -2230,7 +2230,7 @@ void statusline(message_type importance, const char *msg, ...)
 	va_end(ap);
 
 #ifdef ENABLE_MULTIBUFFER
-	if (!we_are_running && importance == ALERT && openfile &&
+	if (!we_are_running && importance == ALERT && openfile && !openfile->fmt &&
 						!openfile->errormessage && openfile->next != openfile)
 		openfile->errormessage = copy_of(compound);
 #endif