Fix compiler warnings:

Fix compiler warnings:

pp_sys.c: In function `Perl_pp_tie':
pp_sys.c:788: warning: 'gv' might be used uninitialized in this function
pp_sys.c: In function `Perl_pp_dbmopen':
pp_sys.c:936: warning: 'gv' might be used uninitialized in this function
pp_sys.c: In function `Perl_pp_enterwrite':
pp_sys.c:1274: warning: 'cv' might be used uninitialized in this function
pp_sys.c: In function `Perl_pp_ghostent':
pp_sys.c:4711: warning: 'hent' might be used uninitialized in this function