bc: unbreak read(), remove checks for nested read()

bc: unbreak read(), remove checks for nested read()

function                                             old     new   delta
zbc_parse_pushSTR                                      -      65     +65
common_parse_expr                                      -      32     +32
bc_error_nested_read_call                             10       -     -10
bc_parse_expr_empty_ok                              1977    1963     -14
zdc_parse_expr                                       635     615     -20
zcommon_parse_expr                                    32       -     -32
zbc_program_exec                                    4064    4023     -41
bc_parse_pushSTR                                      65       -     -65
------------------------------------------------------------------------------
(add/remove: 2/3 grow/shrink: 0/3 up/down: 97/-182)           Total: -85 bytes
   text	   data	    bss	    dec	    hex	filename
 981661	    485	   7296	 989442	  f1902	busybox_old
 981540	    485	   7296	 989321	  f1889	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>