shell/vfs: Handle print failure in genfile cmd gracefully

sys/shell/cmds/vfs.c

@@ -744,7 +744,19 @@ static void _write_block(int fd, unsigned bs, unsigned i)
     char block[bs];
     char *buf = block;
 
-    buf += snprintf(buf, bs, "|%03u|", i);
+    int size_wanted = snprintf(buf, bs, "|%03u|", i);
+
+    if (size_wanted < 0) {
+        assert(0);
+        return;
+    }
+
+    /* Did the output got truncated? */
+    if ((unsigned) size_wanted >= bs) {
+        buf += bs;
+    } else {
+        buf += size_wanted;
+    }
 
     memset(buf, _get_char(i), &block[bs] - buf);
     block[bs - 1] = '\n';