gawk NF=-2 not always caught

[Dan Jacobson]

Making NF negative isn't always caught and the error messages could be
clearer.

$ echo|gawk '{NF=-2}'
$ echo|gawk '{NF=-2;print}'

$ echo|gawk '{NF-=2;print}'

$ echo o|gawk '{NF-=2;print}'
gawk: field.c:145: rebuild_record: Assertion `NF != -1' failed.
Aborted

==========
awk: cmd. line:7: (FILENAME=- FNR=1) fatal error: internal error
p: line 15:  8976 Aborted                 awk '
function shift(p) { #optional p: how many to shift by, like the shell
  if (p=="")p=1;else if (p<0) exit 33 #should print error message
  for(i=1;i<=NF;i++){
    $i=$(i+p)}
  NF-=p #bye bye field separator too
}
{ #testing:
for (m=1;m<=10;m++){
print
shift(4)
}}
'  <<!
a a 
!

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