ecpg: fix some minor mishandling of bad input in preprocessor.

ecpg: fix some minor mishandling of bad input in preprocessor.

Avoid null-pointer crash when considering a cursor declaration
that's outside any C function (a case which is useless anyway).

Ensure a cursor for a prepared statement is marked as initially
not open.  At worst, if we chanced to get not-already-zeroed memory
from malloc(), this oversight would result in failing to issue a
"cursor "foo" has been declared but not opened" warning that would
have been appropriate.

Avoid running off the end of the buffer when there are mismatched
square brackets following a variable name.  This could lead to
SIGSEGV after reaching the end of memory.

Given the lack of field complaints, none of these seem to be worth
back-patching, but let's clean them up in HEAD.

Per valgrind testing by Alexander Lakhin.

Discussion: https://postgr.es/m/5f5bcecd-d7ec-b8c0-6c92-d1a7c6e0f639@gmail.com