[damned-lies] Prevent crashing when new_branch_release is blank in 'new branch' form

[Claude Paroz <claudep src gnome org>]

commit 9d684e97cd1140062bc2a6618046318d6fa8b1bd
Author: Claude Paroz <claude 2xlibre net>
Date:   Thu Mar 16 14:51:29 2017 +0100

    Prevent crashing when new_branch_release is blank in 'new branch' form

 stats/forms.py |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)
---
diff --git a/stats/forms.py b/stats/forms.py
index 1ec5828..71bf49a 100644
--- a/stats/forms.py
+++ b/stats/forms.py
@@ -70,8 +70,17 @@ class ModuleBranchForm(forms.Form):
             except Branch.DoesNotExist:
                 # The branch will be created later in the view
                 return cleaned_data
-            rel = Release.objects.get(pk=cleaned_data['new_branch_release'].pk)
-            cat = Category(release=rel, branch=branch, name=cleaned_data['new_branch_category'])
-            cat.full_clean()
-            cleaned_data['new_category'] = cat
+            if not cleaned_data['new_branch_release']:
+                self.add_error(
+                    'new_branch_release',
+                    ValidationError(
+                        "There is already an entry for branch %s. Edit that one if "
+                        "you want to remove the release link." % branch
+                    )
+                )
+            else:
+                rel = Release.objects.get(pk=cleaned_data['new_branch_release'].pk)
+                cat = Category(release=rel, branch=branch, name=cleaned_data['new_branch_category'])
+                cat.full_clean()
+                cleaned_data['new_category'] = cat
         return cleaned_data