markdown: Prevent OverflowError with large time integers.

`<time:1234567890123>` causes a "signed integer is greater than
maximum" exception from dateutil.parser; datetime also cannot handle
it ("year 41091 is out of range") but that is a ValueError which is
already caught.

Catch the OverflowError thrown by dateutil.

zerver/lib/markdown/__init__.py

@@ -1395,7 +1395,7 @@ class Timestamp(markdown.inlinepatterns.Pattern):
         time_input_string = match.group("time")
         try:
             timestamp = dateutil.parser.parse(time_input_string, tzinfos=common_timezones)
-        except ValueError:
+        except (ValueError, OverflowError):
             try:
                 timestamp = datetime.fromtimestamp(float(time_input_string), tz=timezone.utc)
             except ValueError:

zerver/tests/fixtures/markdown_test_cases.json

@@ -844,6 +844,13 @@
       "expected_output": "<p>Let's meet at <time datetime=\"2017-06-05T22:30:00Z\">1496701800</time>.</p>",
       "text_content": "Let's meet at 1496701800."
     },
+    {
+      "name": "timestamp_unix_overflow",
+      "input": "Let's meet at <time:1234567890123>.",
+      "expected_output": "<p>Let's meet at <span class=\"timestamp-error\">Invalid time format: 1234567890123</span>.</p>",
+      "marked_expected_output": "<p>Let's meet at <time datetime=\"+041091-11-25T05:02:03Z\">1234567890123</time>.</p>",
+      "text_content": "Let's meet at Invalid time format: 1234567890123."
+    },
     {
       "name": "tex_inline",
       "input": "$$1 \\oplus 0 = 1$$",