Prevent NPE in CommentMustAsDiff if no hunk header (#1519) (#15201)

Backport #15199

I do not understand how this can happen or why.

There is an apparent possibility for a comment.Patch to be missing a hunk header
- this should not happen and do not understand how. But it appears to happen on
1.13 at least in some case.

This PR will simply add a new section if the cursection is empty
thus preventing the NPE.

Fix #15198

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/gitdiff/gitdiff.go

@@ -1014,6 +1014,11 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 			}
 			diffLine := &DiffLine{Type: DiffLineAdd, RightIdx: rightLine}
 			rightLine++
+			if curSection == nil {
+				// Create a new section to represent this hunk
+				curSection = &DiffSection{}
+				curFile.Sections = append(curFile.Sections, curSection)
+			}
 			curSection.Lines = append(curSection.Lines, diffLine)
 		case '-':
 			curFileLinesCount++
@@ -1026,6 +1031,11 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 			if leftLine > 0 {
 				leftLine++
 			}
+			if curSection == nil {
+				// Create a new section to represent this hunk
+				curSection = &DiffSection{}
+				curFile.Sections = append(curFile.Sections, curSection)
+			}
 			curSection.Lines = append(curSection.Lines, diffLine)
 		case ' ':
 			curFileLinesCount++
@@ -1036,6 +1046,11 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 			diffLine := &DiffLine{Type: DiffLinePlain, LeftIdx: leftLine, RightIdx: rightLine}
 			leftLine++
 			rightLine++
+			if curSection == nil {
+				// Create a new section to represent this hunk
+				curSection = &DiffSection{}
+				curFile.Sections = append(curFile.Sections, curSection)
+			}
 			curSection.Lines = append(curSection.Lines, diffLine)
 		default:
 			// This is unexpected