GH271: Warning on </dev/null to CA.pl

If CA.pl is reading from /dev/null, then "chop $FILE" gives a warning.
Sigh.  Have to add "if $FILE".  This just silences a build warning.
Thanks to GitHub user andrejs-igumenovs for help with this.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

apps/CA.pl.in

@@ -121,7 +121,7 @@ if ($WHAT eq '-newcert' ) {
     # ask user for existing CA certificate
     print "CA certificate filename (or enter to create)\n";
     $FILE = <STDIN>;
-    chop $FILE;
+    chop $FILE if $FILE;
     if ($FILE) {
         copy_pemfile($FILE,"${CATOP}/private/$CAKEY", "PRIVATE");
         copy_pemfile($FILE,"${CATOP}/$CACERT", "CERTIFICATE");