Add test to improve code coverage.

(cherry picked from commit 5703dbc)

test/data/coverage_xmp_doctype.exv

@@ -0,0 +1,9 @@
+<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
+ <!DOCTYPE foo [  
+   <!ELEMENT foo ANY >
+   <!ENTITY xxe SYSTEM "file:///dev/random" >]>
+<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 4.4.0-Exiv2">
+<foo>
+</foo>
+</x:xmpmeta>
+<?xpacket end="w"?>

tests/bugfixes/github/test_coverage_xmp_doctype.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+
+from system_tests import CaseMeta, path, check_no_ASAN_UBSAN_errors
+
+class coverage_xmp_doctype(metaclass=CaseMeta):
+    """
+    Test added to improve code coverage in xmpsidecar.cpp after
+    Codecov complained about a lack of code coverage in this PR:
+    https://github.com/Exiv2/exiv2/pull/1878
+    """
+
+    filename = path("$data_path/coverage_xmp_doctype.exv")
+    commands = ["$exiv2 $filename"]
+    stderr = ["""Error: XMP Toolkit error 201: Error in XMLValidator
+Warning: Failed to decode XMP metadata.
+$filename: No Exif data found in the file
+"""]
+    retval = [253]
+
+    compare_stdout = check_no_ASAN_UBSAN_errors