This is a patch to fix crashes in psql when executing queries from
an external file. The code also adds error checking to verify that
memory for "query" was allocated. The conditional for the block of
code was changed from "query == NULL" to "query_alloced == false".

The conditional, "query == NULL", was never true. This prevented
the memory being allocated for "query". A few lines later, an attempt
to write to an un-allocated memory area generated a SIGSEGV causing
the frontend to crash.