expand: avoid a panic when ParamExp.Repl.Orig is nil

Pattern expected the word to never be nil, unlike other APIs
such as Literal or Document. Fix that to avoid other similar panics.

And when encountering a nil or empty ParamExp.Repl.Orig,
do not perform a search and replace, as it would result in every match.

Fixes #1076.

expand/expand.go

@@ -214,6 +214,9 @@ const patMode = pattern.Filenames | pattern.Braces
 // The config specifies shell expansion options; nil behaves the same as an
 // empty config.
 func Pattern(cfg *Config, word *syntax.Word) (string, error) {
+	if word == nil {
+		return "", nil
+	}
 	cfg = prepareConfig(cfg)
 	field, err := cfg.wordField(word.Parts, quoteNone)
 	if err != nil {

expand/param.go

@@ -206,6 +206,9 @@ func (cfg *Config) paramExp(pe *syntax.ParamExp) (string, error) {
 		if err != nil {
 			return "", err
 		}
+		if orig == "" {
+			break // nothing to replace
+		}
 		with, err := Literal(cfg, pe.Repl.With)
 		if err != nil {
 			return "", err

interp/interp_test.go

@@ -509,6 +509,7 @@ var runTests = []runTest{
 	{"a='abcx1y'; echo ${a//x[[:digit:]]y}", "abc\n"},
 	{`a=xyz; echo "${a/y/a  b}"`, "xa  bz\n"},
 	{"a='foo_interp_missing/bar_interp_missing'; echo ${a//o*a/}", "fr_interp_missing\n"},
+	{"a=foobar; echo ${a//a/} ${a///b} ${a///}", "foobr foobar foobar\n"},
 	{
 		"echo ${a:-b}; echo $a; a=; echo ${a:-b}; a=c; echo ${a:-b}",
 		"b\n\nb\nc\n",