pgsql: Avoid unnecessary use of strncpy in a couple of places in ecpg.

Avoid unnecessary use of strncpy in a couple of places in ecpg.

Use of strncpy with a length limit based on the source, rather than
the destination, is non-idiomatic and draws warnings from gcc 8.
Replace with memcpy, which does exactly the same thing in these cases,
but with less chance for confusion.

Backpatch to all supported branches.

Discussion: https://postgr.es/m/21789.1529170195@sss.pgh.pa.us

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8c924855456eff392391eba49cb02ae581a12898

Modified Files
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src/interfaces/ecpg/ecpglib/descriptor.c | 2 +-
src/interfaces/ecpg/pgtypeslib/common.c  | 6 ++----
2 files changed, 3 insertions(+), 5 deletions(-)