Don't call open("") (#1336)

strings.Split(s, sep) returns a slice of a single element containing s
if sep is not found in s. This is true even if s is empty.

As a result, every call to flagFromEnvOrFile results in an attempt to
open a file with empty name. This is seen from strace as

[pid 3287620] openat(AT_FDCWD, "", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 3287620] openat(AT_FDCWD, "", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
...

To fix, check if the string is empty before calling ReadFile.

This also fixes cases where filePath is non-empty but has extra commas.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

flag.go

@@ -434,8 +434,10 @@ func flagFromEnvOrFile(envVars []string, filePath string) (val string, ok bool)
 		}
 	}
 	for _, fileVar := range strings.Split(filePath, ",") {
-		if data, err := ioutil.ReadFile(fileVar); err == nil {
-			return string(data), true
+		if fileVar != "" {
+			if data, err := ioutil.ReadFile(fileVar); err == nil {
+				return string(data), true
+			}
 		}
 	}
 	return "", false