Fix out-of-bounds reads in ParseRfc822Date() with too short input

The implementation implicitly relies, in many places, on the assumption
that the input never ends prematurely. If it does, the iterator
pointing beyond the end of buffer is dereferenced, which is UB.

The solution used here is to append 32 zero bytes to the date string,
which hopefully keeps the code more readable than checking for the end
of string before each deference operation.

Add various syntactically invalid inputs to unit tests.

Closes #22185.

tests/datetime/datetimetest.cpp

@@ -1099,6 +1099,54 @@ void DateTimeTestCase::TestParseRFC822()
             { 0 },
             false
         },
+
+        {
+            "Sun, 01 Und 2008 16:30:10 +0200", // month: Undecimber
+            { 0 },
+            false
+        },
+
+        {
+            "Sun, 01 Jun 2008 16:3:10 +0200", // missing digit
+            { 0 },
+            false
+        },
+
+        {
+            "Sun 01 Jun 2008 16:39:10 +0200", // missing comma
+            { 0 },
+            false
+        },
+
+        {
+            "Sat, 18 Dec 1999 10:48:30", // TZ missing
+            { 0 },
+            false
+        },
+
+        {
+            "Sat, 18 Dec 1999", // time missing
+            { 0 },
+            false
+        },
+
+        {
+            "Sat, 18 Dec 2", // most of year missing
+            { 0 },
+            false
+        },
+
+        {
+            "Sun,", // missing date and time
+            { 0 },
+            false
+        },
+
+        {
+            "", // empty input
+            { 0 },
+            false
+        },
     };
 
     wxGCC_WARNING_RESTORE(missing-field-initializers)