dpkg-maintscript-helper: Allow missing prior-version for dir/symlink switches

Closes: #733980

debian/changelog

@@ -49,6 +49,8 @@ dpkg (1.17.6) UNRELEASED; urgency=low
     with the new dpkg-deb option --uniform-compression.
   * Fix file descriptor leaks in diversions and statoverride databases.
     Closes: #734783
+  * Allow missing prior-version argument in dpkg-maintscript-helper
+    dir_to_symlink and symlink_to_dir commands. Closes: #733980
 
   [ Updated dpkg translations ]
   * Swedish (Peter Krefting).

scripts/dpkg-maintscript-helper.sh

@@ -254,7 +254,6 @@ symlink_to_dir() {
 	[ -n "$PACKAGE" ] || error "cannot identify the package"
 	[ -n "$SYMLINK" ] || error "symlink parameter is missing"
 	[ -n "$SYMLINK_TARGET" ] || error "original symlink target is missing"
-	[ -n "$LASTVERSION" ] || error "last version is missing"
 	[ -n "$1" ] || error "maintainer script parameters are missing"
 
 	debug "Executing $0 symlink_to_dir in $DPKG_MAINTSCRIPT_NAME" \
@@ -328,7 +327,6 @@ dir_to_symlink() {
 	[ -n "$PACKAGE" ] || error "cannot identify the package"
 	[ -n "$PATHNAME" ] || error "directory parameter is missing"
 	[ -n "$SYMLINK_TARGET" ] || error "new symlink target is missing"
-	[ -n "$LASTVERSION" ] || error "last version is missing"
 	[ -n "$1" ] || error "maintainer script parameters are missing"
 
 	debug "Executing $0 dir_to_symlink in $DPKG_MAINTSCRIPT_NAME" \