fileserver: reject non-integer browse file_limit (#7988)

caddytest/integration/caddyfile_adapt/file_server_file_limit_not_integer.caddyfiletest

@@ -0,0 +1,9 @@
+:80
+
+file_server {
+	browse {
+		file_limit notanint
+	}
+}
+----------
+file_limit should have an integer value

caddytest/integration/caddyfile_adapt/file_server_file_limit_partial.caddyfiletest

@@ -0,0 +1,9 @@
+:80
+
+file_server {
+	browse {
+		file_limit 12abc
+	}
+}
+----------
+file_limit should have an integer value

modules/caddyhttp/fileserver/caddyfile.go

@@ -139,7 +139,10 @@ func (fsrv *FileServer) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 					if len(fileLimit) != 1 {
 						return d.Err("file_limit should have an integer value")
 					}
-					val, _ := strconv.Atoi(fileLimit[0])
+					val, err := strconv.Atoi(fileLimit[0])
+					if err != nil {
+						return d.Err("file_limit should have an integer value")
+					}
 					if fsrv.Browse.FileLimit != 0 {
 						return d.Err("file_limit is already enabled")
 					}