double free or corruption in python bindings on make test

[strk]

On make test:

*** Running visual tests...
*** glibc detected *** python: double free or corruption (!prev): 0x00000000017a0fe0 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7eb96)[0x2b80cff77b96]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN5boost20checked_array_deleteIjEEvPT_+0x1f)[0x2b80d19c9ec1]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN5boost12scoped_arrayIjED1Ev+0x1b)[0x2b80d19c902f]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN6mapnik8grid2utfINS_8hit_gridIiEEEEvRKT_RN5boost6python4listERSt6vectorINS3_11lookup_typeESaISB_EEj+0x421)[0x2b80d19c6841]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN6mapnik15grid_encode_utfINS_8hit_gridIiEEEEvRKT_RN5boost6python4dictEbj+0x72)[0x2b80d19c5a42]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN6mapnik11grid_encodeINS_8hit_gridIiEEEEN5boost6python4dictERKT_RKSsbj+0x7e)[0x2b80d19c5545]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN5boost6python6detail6invokeINS0_15to_python_valueIRKNS0_4dictEEEPFS4_RKN6mapnik8hit_gridIiEERKSsbjENS0_15arg_from_pythonISC_EENSH_ISE_EENSH_IbEENSH_IjEEEEP7_objectNS1_11invoke_tag_ILb0ELb0EEERKT_RT0_RT1_RT2_RT3_RT4_+0x81)[0x2b80d1788f27]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN5boost6python6detail12caller_arityILj4EE4implIPFNS0_4dictERKN6mapnik8hit_gridIiEERKSsbjENS0_21default_call_policiesENS_3mpl7vector5IS5_SA_SC_bjEEEclEP7_objectSL_+0x1e7)[0x2b80d1787923]
/usr/lib/python2.7/dist-packages/mapnik/_mapnik.so(_ZN5boost6python7objects23caller_py_function_implINS0_6detail6callerIPFNS0_4dictERKN6mapnik8hit_gridIiEERKSsbjENS0_21default_call_policiesENS_3mpl7vector5IS5_SA_SC_bjEEEEEclEP7_objectSM_+0x2f)[0x2b80d1786d4b]

[strk]

The above is as of 71cca69

[springmeyer]

duplicate of #1584