Avoid potential buffer overflow crash

A pointer to a C string was treated as a pointer to a "name" datum and
passed to SPI_execute_plan().  This pointer would then end up being
passed through datumCopy(), which would try to copy the entire 64 bytes
of name data, thus running past the end of the C string.  Fix by
converting the string to a proper name structure.

Found by LLVM AddressSanitizer.
Fix thinko in SPI_execute_plan() calls

Two call sites were apparently thinking that the last argument of
SPI_execute_plan() is the number of query parameters, but it is actually
the row limit.  Change the calls to 0, since we don't care about the
limit there.  The previous code didn't break anything, but it was still
wrong.