Fix inverted bounds check dropping every Erlang string with a backslash (#5110)

parseErlangString advances past a backslash escape and then checks
len(data) >= *i before reading the escaped byte. That condition is
almost always true (it only turns false once *i runs off the end),
so the intended out-of-range guard fires on the very first escape
character it sees instead of only at EOF. Any rebar.lock or OTP
resource file containing a backslash in a quoted string (a Windows
git path, an escaped quote, anything) fails to parse and the whole
file, and every package in it, gets dropped.

Flip the comparison to *i >= len(data) so the guard only trips when
the escape is genuinely truncated, and add a regression test for a
string with an escaped quote.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

syft/pkg/cataloger/erlang/erlang_parser.go

@@ -156,7 +156,7 @@ func parseErlangString(data []byte, i *int) (erlangNode, error) {
 		}
 		if c == '\\' {
 			*i++
-			if len(data) >= *i {
+			if *i >= len(data) {
 				return node(nil), fmt.Errorf("invalid escape without closed string at %d", *i)
 			}
 			c = data[*i]

syft/pkg/cataloger/erlang/erlang_parser_test.go

@@ -96,6 +96,11 @@ func Test_parseErlang(t *testing.T) {
 	{ foo, bar }
 ]}`,
 		},
+		{
+			name: "string with an escaped quote",
+			content: `
+{escaped, ["a\"b"]}`,
+		},
 	}
 
 	for _, test := range tests {
@@ -112,3 +117,14 @@ func Test_parseErlang(t *testing.T) {
 		})
 	}
 }
+
+func Test_parseErlangString_escapedByte(t *testing.T) {
+	// a backslash escape mid-string used to always error out, since the bounds
+	// check at the escape site was inverted (len(data) >= *i is true for
+	// almost every position, not just an out-of-range one).
+	data := []byte(`"a\"b"`)
+	i := 0
+	got, err := parseErlangString(data, &i)
+	require.NoError(t, err)
+	assert.Equal(t, `a"b`, got.String())
+}