Don't throw a spurious error when using DBdeltree

The function call ast_db_deltree returns the number of row deleted, or a
negative number if it failed. DBdeltree was treating any non-zero return
as an error, causing a spurious verbose error message to be displayed.

This patch handles the return code of ast_db_deltree correctly.

(closes issue ASTERISK-21070)
Reported by: ianc
patches:
  dbdeltree.diff uploaded by ianc (License #5955)
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Merged revisions 381364 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 381365 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@381366 f38db490-d61c-443f-a65b-d21fe96a405b