common/cmd_pxe.c: Fix GCC 4.6 build warnings

Fix:
cmd_pxe.c: In function 'parse_pxefile_top':
cmd_pxe.c:941:5: warning: 'err' may be used uninitialized in this
  function [-Wuninitialized]
cmd_pxe.c:921:6: note: 'err' was declared here

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Jason Hobbs <jason.hobbs@calxeda.com>
Acked-by: Jason Hobbs <jason.hobbs@calxeda.com>