Undefined behaviour in gserialized_spgist_picksplit_nd (CREATE INDEX)

[Algunenano]

Detected running test regress_spgist_index_nd with -fsanitize=undefined -fsanitize-undefined-trap-on-error:

Core was generated by `postgres: raul postgis_reg [local] CREATE IN'.
Program terminated with signal SIGILL, Illegal instruction.
#0  0x00007ff58cf98e8f in gserialized_spgist_picksplit_nd (fcinfo=0x7ffc205e4f60) at gserialized_spgist_nd.c:335
335                             if (GIDX_GET_MAX(box, i) != FLT_MAX)
(gdb) bt
#0  0x00007ff58cf98e8f in gserialized_spgist_picksplit_nd (fcinfo=0x7ffc205e4f60) at gserialized_spgist_nd.c:335
#1  0x000056216c80ee51 in FunctionCall2Coll (flinfo=0x7ff5f56a515c, collation=1, arg1=33, arg2=33) at fmgr.c:1145
#2  0x000056216c3f07c9 in doPickSplit (index=<optimized out>, state=<optimized out>, current=<optimized out>, parent=<optimized out>, 
    newLeafTuple=<optimized out>, level=<optimized out>, isNulls=<optimized out>, isNew=<optimized out>) at spgdoinsert.c:829
#3  spgdoinsert (index=<optimized out>, state=<optimized out>, heapPtr=0x56216dd7cefc, datum=140694363145508, isnull=false)
    at spgdoinsert.c:2077
#4  0x000056216c3ec6c9 in spgistBuildCallback (index=0x7ff58d1e31f0, htup=<optimized out>, values=0x7ffc205e58e0, isnull=0x7ffc205e58c0, 
    tupleIsAlive=<optimized out>, state=0x7ffc205e5a80) at spginsert.c:57
#5  0x000056216c4374fe in IndexBuildHeapRangeScan (heapRelation=0x7ff58d1d0d30, indexRelation=0x7ff58d1e31f0, indexInfo=0x56216dd7c950, 
    allow_sync=<optimized out>, anyvisible=false, start_blockno=<optimized out>, numblocks=4294967295, 
    callback=0x56216c3ec680 <spgistBuildCallback>, callback_state=0x7ffc205e5a80, scan=0x56216dd7ceb0) at index.c:2944
#6  0x000056216c436d53 in IndexBuildHeapScan (heapRelation=0x56216dcf6401, indexRelation=0x1, indexInfo=0x21, allow_sync=33, 
    callback=0x7ff5f56a515c, callback_state=0x56216dbcd010, scan=0x0) at index.c:2458
}}

Does https://github.com/postgis/postgis/blob/126b97b077a1ee512a6de71931fb04de58546154/postgis/gserialized_spgist_nd.c#L335 need to use `j` instead of `i`? If so, let's consider using a more descriptive name (`tupleIterator`, `dimension`...).

Related to #4230

Tested with PG11 and Postgis trunk.

[Algunenano]

WIP with C99 sugar in ​https://github.com/postgis/postgis/pull/331

[algunenano]

In 17033:

Fix undefined behaviour in gserialized_spgist_picksplit_nd

Closes #4233
Closes ​https://github.com/postgis/postgis/pull/331