fix: Fix passing NULL to memcpy when unescaping a CRS value

[paleolimbot]

Found by the CRAN check: https://www.stats.ox.ac.uk/pub/bdr/memtests/gcc-UBSAN/geoarrow/tests/testthat.Rout

geoarrow.c:4843:7: runtime error: null pointer passed as argument 1, which is declared to never be null
    #0 0x7f0fcf929ecf in GeoArrowUnescapeCrs /data/gannet/ripley/R/packages/tests-gcc-SAN/geoarrow/src/geoarrow.c:4843
    #1 0x7f0fcf9e453b in geoarrow_c_schema_parse /data/gannet/ripley/R/packages/tests-gcc-SAN/geoarrow/src/r-type.c:82
    #2 0x58c2e5 in R_doDotCall /data/gannet/ripley/R/svn/R-devel/src/main/dotcode.c:757
    #3 0x652927 in bcEval_loop /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:8690
    #4 0x68d632 in bcEval /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:7523