pgsql: Improve coding of column-name parsing in psql's new crosstabview

Improve coding of column-name parsing in psql's new crosstabview.c.

Coverity complained about this code, not without reason because it was
rather messy.  Adjust it to not scribble on the passed string; that adds
one malloc/free cycle per column name, which is going to be insignificant
in context.  We can actually const-ify both the string argument and the
PGresult.

Daniel Verité, with some further cleanup by me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7a5f8b5c59033ac153963f98b9109be9529a824a

Modified Files
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src/bin/psql/crosstabview.c | 64 +++++++++++++++++++++++++++------------------
src/bin/psql/crosstabview.h |  3 ++-
2 files changed, 40 insertions(+), 27 deletions(-)