syntax: error on unclosed nested backquotes

Nested backquote command substitutions use backslashes, such as:

	` \`echo foo\` `

When looking for the closing backquote, we didn't error if the number of
backslashes in the closing token was lower than expected. Do that, and
add a regression test.

Fixes mvdan#349.

syntax/parser.go

@@ -1003,6 +1003,11 @@ func (p *Parser) wordPart() WordPart {
 
 		p.next()
 		cs.StmtList = p.stmtList()
+		if p.tok == bckQuote && p.lastBquoteEsc < p.openBquotes-1 {
+			// e.g. found ` before the nested backquote \` was closed.
+			p.tok = _EOF
+			p.quoteErr(cs.Pos(), bckQuote)
+		}
 		p.postNested(old)
 		p.openBquotes--
 		cs.Right = p.pos

syntax/parser_test.go

@@ -1188,6 +1188,10 @@ var shellTests = []errorCase{
 		in:     "`\"`",
 		common: "1:3: reached EOF without closing quote `",
 	},
+	{
+		in:     "`\\```",
+		common: "1:2: reached EOF without closing quote `",
+	},
 	{
 		in:     "`{\n`",
 		common: "1:2: reached ` without matching { with }",