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

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

Don't get confused by an unmatched right brace in the input.
(Previously, this led to discarding information about file-level
variables and then possibly crashing.)

Detect, rather than crash on, an attempt to index into a non-array
variable.

As before, in the absence of field complaints I'm not too
excited about back-patching these.

Per valgrind testing by Alexander Lakhin.

Discussion: https://postgr.es/m/a239aec2-6c79-5fc9-9272-cea41158a360@gmail.com