LibPDF: Don't consider the End of Data code as normal ASCII85 input

Data encoded with ASCII85 is terminated with the EOD code 0x7E3E. This
should not be considered as normal input but rather discarded.

Userland/Libraries/LibPDF/Filter.cpp

@@ -94,6 +94,8 @@ PDFErrorOr<ByteBuffer> Filter::decode_ascii_hex(ReadonlyBytes bytes)
 
 PDFErrorOr<ByteBuffer> Filter::decode_ascii85(ReadonlyBytes bytes)
 {
+    // 3.3.2 ASCII85Decode Filter
+
     ByteBuffer buffer;
     TRY(buffer.try_ensure_capacity(bytes.size()));
 
@@ -114,10 +116,21 @@ PDFErrorOr<ByteBuffer> Filter::decode_ascii85(ReadonlyBytes bytes)
 
         u32 number = 0;
 
-        auto const to_write = byte_index + 5 >= bytes.size() ? bytes.size() - byte_index : 5;
+        auto to_write = byte_index + 5 >= bytes.size() ? bytes.size() - byte_index : 5;
+
+        Optional<u32> end_of_data_index {};
 
         for (int i = 0; i < 5; i++) {
-            auto byte = byte_index >= bytes.size() ? 'u' : bytes[byte_index++];
+            // We check for the EOD sequence '~>', but as '~' can only appear in
+            // this sequence, there is no need to check for '>'.
+            if (!end_of_data_index.has_value() && bytes[byte_index] == '~') {
+                end_of_data_index = i;
+                to_write = i + 1;
+            }
+
+            bool const should_fake_end = byte_index >= bytes.size() || end_of_data_index.has_value();
+            auto const byte = should_fake_end ? 'u' : bytes[byte_index++];
+
             if (Reader::is_whitespace(byte)) {
                 i--;
                 continue;
@@ -127,6 +140,9 @@ PDFErrorOr<ByteBuffer> Filter::decode_ascii85(ReadonlyBytes bytes)
 
         for (size_t i = 0; i < to_write - 1; i++)
             buffer.append(reinterpret_cast<u8*>(&number)[3 - i]);
+
+        if (end_of_data_index.has_value())
+            break;
     }
 
     return buffer;