Fix out-of-bounds array index (#597)

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>

prost-build/src/code_generator.rs

@@ -963,7 +963,7 @@ fn unescape_c_escape_string(s: &str) -> Vec<u8> {
                     dst.push(octal);
                 }
                 b'x' | b'X' => {
-                    if p + 2 > len {
+                    if p + 3 > len {
                         panic!(
                             "invalid c-escaped default binary value ({}): incomplete hex value",
                             s
@@ -1074,6 +1074,12 @@ mod tests {
         );
     }
 
+    #[test]
+    #[should_panic(expected = "incomplete hex value")]
+    fn test_unescape_c_escape_string_incomplete_hex_value() {
+        unescape_c_escape_string(r#"\x1"#);
+    }
+
     #[test]
     fn test_strip_enum_prefix() {
         assert_eq!(strip_enum_prefix("Foo", "FooBar"), "Bar");