Fix opaque panic on invariant violation in ApplyTarPatch.

To reproduce, use PUT to upload this archive (`unzstd | base64 -d`):

KLUv/QRY7QIAxAJhL2IAMDAwMDY0NDAwMDAwMDEAADAwNzU2MAAgMAB1c3RhcgAwAGEAMzM3
YREA/UEF/EC9Y0AdDJBP8GDCTaDGBxATkAAd3gJoMPAbJANAciACGDTAsXKZngAR/m3nXA==

then issue any PATCH request to that site.

After this commit, the server returns "malformed manifest (not
a directory)" instead of "assignment to entry in nil map".

While ideally incoming manifests should be checked for consistency
regardless of how they're uploaded, in practice this is only a self-DoS
so it's probably not worth fixing.

V12-Ref: F-77244

src/patch.go

@@ -52,13 +52,16 @@ func ApplyTarPatch(manifest *Manifest, reader io.Reader, parents CreateParentsMo
 			iter := root
 			for _, segment := range segments[:len(segments)-1] {
 				if iter.children == nil {
-					panic("malformed manifest (not a directory)")
+					break // error handled below
 				} else if _, exists := iter.children[segment]; !exists {
 					panic("malformed manifest (node does not exist)")
 				} else {
 					iter = iter.children[segment]
 				}
 			}
+			if iter.children == nil {
+				panic("malformed manifest (not a directory)")
+			}
 			iter.children[fileName] = node
 		}
 	}