check input length for librnp 0.18

[remicollet]

To preserve extension behavior on empty input

Using librnp 0.18 without this

========DIFF========
001- bool(false)
001+ string(15) "{
002+   "keys":[]
003+ }"
     string(864) "{
       "keys":[
         {
--
========DONE========
FAIL import keys test [tests/016.phpt] 

========DIFF========
001- bool(false)
001+ string(13) ":empty input
002+ "
     bool(false)
     bool(false)
     bool(false)
--
========DONE========
FAIL Dump packets test [tests/006.phpt] 

With this PR

=====================================================================
TEST RESULT SUMMARY
---------------------------------------------------------------------
Exts skipped    :     0
Exts tested     :    17
---------------------------------------------------------------------

Number of tests :    18                18
Tests skipped   :     0 (  0.0%) --------
Tests warned    :     0 (  0.0%) (  0.0%)
Tests failed    :     0 (  0.0%) (  0.0%)
Tests passed    :    18 (100.0%) (100.0%)
---------------------------------------------------------------------
Time taken      :     4 seconds
=====================================================================