Fix parsing invalid intervals

Parsing string with slash at the start or the end raised IndexError
instead of ParseError.

src/pendulum/parsing/__init__.py

@@ -214,11 +214,11 @@ def _parse_iso8601_interval(text: str) -> _Interval:
     first, last = text.split("/")
     start = end = duration = None
 
-    if first[0] == "P":
+    if first[0:1] == "P":
         # duration/end
         duration = parse_iso8601(first)
         end = parse_iso8601(last)
-    elif last[0] == "P":
+    elif last[0:1] == "P":
         # start/duration
         start = parse_iso8601(first)
         duration = parse_iso8601(last)

tests/parsing/test_parsing.py

@@ -660,6 +660,16 @@ def test_invalid():
     with pytest.raises(ParserError):
         parse(text)
 
+    text = "/2012"
+
+    with pytest.raises(ParserError):
+        parse(text)
+
+    text = "2012/"
+
+    with pytest.raises(ParserError):
+        parse(text)
+
 
 def test_exif_edge_case():
     text = "2016:12:26 15:45:28"