der: fix MAX_DER for 32bit targets.

src/der.rs

@@ -242,9 +242,14 @@ const LONG_FORM_LEN_THREE_BYTES_MAX: usize = (1 << (8 * 3)) - 1;
 // Leading octet for long definite form DER length expressed in subsequent four bytes.
 const LONG_FORM_LEN_FOUR_BYTES: u8 = 0x84;
 
+#[cfg(target_pointer_width = "64")]
 // Maximum size that can be expressed in a four byte long form der len.
 const LONG_FORM_LEN_FOUR_BYTES_MAX: usize = (1 << (8 * 4)) - 1;
 
+#[cfg(target_pointer_width = "32")]
+// Maximum size that can be expressed on 32 bit platforms is the usize limit.
+const LONG_FORM_LEN_FOUR_BYTES_MAX: usize = usize::MAX;
+
 // TODO: investigate taking decoder as a reference to reduce generated code
 // size.
 pub(crate) fn nested_of_mut<'a, E>(