awk: be more paranoid when freeing loop variable; make code less obfuscated

awk: be more paranoid when freeing loop variable; make code less obfuscated

function                                             old     new   delta
nvfree                                               149     170     +21
next_token                                           928     932      +4
getvar_i                                              95      94      -1
skip_spaces                                           51      45      -6
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(add/remove: 0/0 grow/shrink: 2/2 up/down: 25/-7)              Total: 18 bytes

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