Get rid of scribbling on a const variable in psql's print.c.
Commit
a2dabf0e1dda93c8 had the bright idea that it could modify a "const"
global variable if it merely casted away const from a pointer. This does
not work on platforms where the compiler puts "const" variables into
read-only storage. Depressingly, we evidently have no such platforms in
our buildfarm ... an oversight I have now remedied. (The one platform
that is known to catch this is recent OS X with -fno-common.)
Per report from Chris Ruprecht. Back-patch to 9.5 where the bogus
code was introduced.