Fix memory leak in lwgeom_grid_in_place

[Algunenano]

Issue arised when collapsing the first ring in a POLYGONTYPE. The other rings
where 'removed' but not freed.

LeakSanitizer: detected memory leaks when running tests in liblwgeom

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x7f08a9e30ae9 in __interceptor_malloc /build/gcc-multilib/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:62
    #1 0x7f08a9a368be in ptarray_construct_empty /home/raul/dev/public/postgis/liblwgeom/ptarray.c:72
    #2 0x7f08a9a7dd1d in wkt_parser_ptarray_new /home/raul/dev/public/postgis/liblwgeom/lwin_wkt.c:306
    #3 0x7f08a9a75101 in wkt_yyparse /home/raul/dev/public/postgis/liblwgeom/lwin_wkt_parse.y:527
    #4 0x7f08a9a77113 in lwgeom_parse_wkt /home/raul/dev/public/postgis/liblwgeom/lwin_wkt_parse.y:68
    #5 0x7f08a9a80964 in lwgeom_from_wkt /home/raul/dev/public/postgis/liblwgeom/lwin_wkt.c:908
    #6 0x55e3818ea500 in do_grid_test /home/raul/dev/public/postgis/liblwgeom/cunit/cu_misc.c:145
    #7 0x55e3818ea794 in test_grid_in_place /home/raul/dev/public/postgis/liblwgeom/cunit/cu_misc.c:193
    #8 0x7f08a95b5087 in run_single_test /tmp/yaourt-tmp-raul/aur-cunit/src/CUnit-2.1-3/CUnit/Sources/Framework/TestRun.c:991

Indirect leak of 128 byte(s) in 1 object(s) allocated from:
    #0 0x7f08a9e30ef9 in __interceptor_realloc /build/gcc-multilib/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:75
    #1 0x7f08a9a3a76d in ptarray_insert_point /home/raul/dev/public/postgis/liblwgeom/ptarray.c:134
    #2 0x7f08a9a3aa4b in ptarray_append_point /home/raul/dev/public/postgis/liblwgeom/ptarray.c:183
    #3 0x7f08a9a7da75 in wkt_parser_ptarray_add_coord /home/raul/dev/public/postgis/liblwgeom/lwin_wkt.c:296
    #4 0x7f08a9a75061 in wkt_yyparse /home/raul/dev/public/postgis/liblwgeom/lwin_wkt_parse.y:525
    #5 0x7f08a9a77113 in lwgeom_parse_wkt /home/raul/dev/public/postgis/liblwgeom/lwin_wkt_parse.y:68
    #6 0x7f08a9a80964 in lwgeom_from_wkt /home/raul/dev/public/postgis/liblwgeom/lwin_wkt.c:908
    #7 0x55e3818ea500 in do_grid_test /home/raul/dev/public/postgis/liblwgeom/cunit/cu_misc.c:145
    #8 0x55e3818ea794 in test_grid_in_place /home/raul/dev/public/postgis/liblwgeom/cunit/cu_misc.c:193
    #9 0x7f08a95b5087 in run_single_test /tmp/yaourt-tmp-raul/aur-cunit/src/CUnit-2.1-3/CUnit/Sources/Framework/TestRun.c:991

SUMMARY: AddressSanitizer: 152 byte(s) leaked in 2 allocation(s).

[Algunenano]

Trac issue: https://trac.osgeo.org/postgis/ticket/3910#ticket

[Algunenano]

First build failed because it was using for (int i = 0; ...) instead of int i; for (i=0; which requires C99. I'm pretty sure it's already an implicit requirement (snprintf, inline functions, variables midblock...) but I've changed it to make the build happy .

[Algunenano]

Committed in https://trac.osgeo.org/postgis/changeset/16020