perl5db: only check sub names for non-references

DB::sub can be called by perl giving it either a name (if it can be
determined) or a code ref. There is special handling for AUTOLOAD subs
and threads::new. This could only be happen when given the name of the
sub, so there is no need to do these checks if given a reference.
Additionally, a reference could be an object, which could have
overloads. Those overloads could fail or otherwise complicate the normal
operation of DB::sub.

Add a ref check to the sub given to DB::sub to avoid these issues.

Author: haarg

lib/perl5db.pl

@@ -4460,13 +4460,13 @@ sub DB::sub {
         # Whether or not the autoloader was running, a scalar to put the
         # sub's return value in (if needed), and an array to put the sub's
         # return value in (if needed).
-        if ($sub eq 'threads::new' && $ENV{PERL5DB_THREADED}) {
+        if (ref $sub && $sub eq 'threads::new' && $ENV{PERL5DB_THREADED}) {
             print "creating new thread\n";
         }
 
         # If the last ten characters are '::AUTOLOAD', note we've traced
         # into AUTOLOAD for $sub.
-        if ( length($sub) > 10 && substr( $sub, -10, 10 ) eq '::AUTOLOAD' ) {
+        if ( ref $sub && length($sub) > 10 && substr( $sub, -10, 10 ) eq '::AUTOLOAD' ) {
             no strict 'refs';
             $al = " for $$sub" if defined $$sub;
         }
@@ -4601,13 +4601,13 @@ sub lsub : lvalue {
         # sub's return value in (if needed), and an array to put the sub's
         # return value in (if needed).
         my ( $al, $ret, @ret ) = "";
-        if ($sub =~ /^threads::new$/ && $ENV{PERL5DB_THREADED}) {
+        if ( ref $sub && $sub =~ /^threads::new$/ && $ENV{PERL5DB_THREADED}) {
             print "creating new thread\n";
         }
 
         # If the last ten characters are C'::AUTOLOAD', note we've traced
         # into AUTOLOAD for $sub.
-        if ( length($sub) > 10 && substr( $sub, -10, 10 ) eq '::AUTOLOAD' ) {
+        if ( ref $sub && length($sub) > 10 && substr( $sub, -10, 10 ) eq '::AUTOLOAD' ) {
             $al = " for $$sub";
         }