Write padding values to ensure pcre2_serialize_encode() outputs defined values (#826)

Fixes low-severity valgrind error reported in GHSA-q7rw-r7qq-2hx6.

Author: NWilson

src/pcre2_compile_class.c

@@ -1802,17 +1802,14 @@ if ((xclass_props & XCLASS_REQUIRED) != 0)
       PUT(code, 0, (uint32_t)(char_lists_size >> 1));
       code += LINK_SIZE;
 
-#if defined PCRE2_DEBUG || defined SUPPORT_VALGRIND
+      /* If we added padding to align the list, initialize the bytes to
+      defined values, so the library is valgrind-clean. It could also
+      be a security concern for clients calling into PCRE2 via bindings
+      from a memory-safe language, if pcre2_serialize_encode() exposes
+      uninitialized memory that may contain sensitive information. */
+
       if ((char_lists_size & 0x2) != 0)
-        {
-        /* In debug the unused 16 bit value is set
-        to a fixed value and marked unused. */
-        ((uint16_t*)data)[-1] = 0x5555;
-#ifdef SUPPORT_VALGRIND
-        VALGRIND_MAKE_MEM_NOACCESS(data - 2, 2);
-#endif
-        }
-#endif
+        ((uint16_t*)data)[-1] = 0xdead;
 
       cb->char_lists_size =
         CLIST_ALIGN_TO(char_lists_size, sizeof(uint32_t));

src/pcre2test_inc.h

@@ -2019,6 +2019,9 @@ uint32_t use_forbid_utf = forbid_utf;
 PCRE2_SIZE patlen, full_patlen;
 PCRE2_SIZE valgrind_access_length;
 PCRE2_SIZE erroroffset;
+int32_t serialize_rc;
+uint8_t *serialized_bytes;
+PCRE2_SIZE serialized_size;
 
 /* The perltest.sh script supports only / as a delimiter. */
 
@@ -2966,6 +2969,28 @@ if ((pat_patctl.control2 & CTL2_NL_SET) != 0)
 rc = show_pattern_info();
 if (rc != PR_OK) return rc;
 
+/* Verify that the compiled structure can be serialized without generating
+memory errors. */
+
+serialize_rc = pcre2_serialize_encode((const pcre2_code **)&compiled_code, 1,
+  &serialized_bytes, &serialized_size, general_context);
+if (serialize_rc != 1)
+  {
+  cfprintf(clr_test_error, outfile, "** pcre2_serialize_encode() returned %d instead of 1\n",
+    serialize_rc);
+  return PR_ABEND;
+  }
+
+#if defined SUPPORT_VALGRIND
+if (VALGRIND_CHECK_MEM_IS_DEFINED(serialized_bytes, serialized_size) != 0)
+  {
+  cfprintf(clr_test_error, outfile, "** pcre2_serialize_encode() returned undefined data\n");
+  return PR_ABEND;
+  }
+#endif
+
+pcre2_serialize_free(serialized_bytes);
+
 /* The "push" control requests that the compiled pattern be remembered on a
 stack. This is mainly for testing the serialization functionality. */