Prevent panic on diff generation (go-gitea#16950)

Backport go-gitea#16950

The lastLeftIdx should be reset at the same time as creating a new section otherwise
it is possible for a second addition to end up attempting to read a nil entry.

Fix go-gitea#16943

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

services/gitdiff/gitdiff.go

@@ -996,6 +996,7 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 
 			// Create a new section to represent this hunk
 			curSection = &DiffSection{}
+			lastLeftIdx = -1
 			curFile.Sections = append(curFile.Sections, curSection)
 
 			lineSectionInfo := getDiffLineSectionInfo(curFile.Name, line, leftLine-1, rightLine-1)
@@ -1036,6 +1037,7 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 				// Create a new section to represent this hunk
 				curSection = &DiffSection{}
 				curFile.Sections = append(curFile.Sections, curSection)
+				lastLeftIdx = -1
 			}
 			if lastLeftIdx > -1 {
 				diffLine.Match = lastLeftIdx
@@ -1061,6 +1063,7 @@ func parseHunks(curFile *DiffFile, maxLines, maxLineCharacters int, input *bufio
 				// Create a new section to represent this hunk
 				curSection = &DiffSection{}
 				curFile.Sections = append(curFile.Sections, curSection)
+				lastLeftIdx = -1
 			}
 			if len(curSection.Lines) == 0 || curSection.Lines[len(curSection.Lines)-1].Type != DiffLineDel {
 				lastLeftIdx = len(curSection.Lines)