pp.c: Make sure variable is initialized

A compiler generated a warning about this.  It is the degenerate case
with an empty input, so isn't really a problem, but silence the warning

1 files changed, 1 insertions, 0 deletions

diff --git a/pp.c b/pp.c
index 14b6c0a223..e283536dc7 100644
--- a/pp.c
+++ b/pp.c
@@ -3486,6 +3486,7 @@ PP(pp_ucfirst)
 
     if (! slen) {   /* If empty */
 	need = 1; /* still need a trailing NUL */
+	ulen = 0;
     }
     else if (DO_UTF8(source)) {	/* Is the source utf8? */
 	doing_utf8 = TRUE;