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

src/interfaces/ecpg/preproc/ecpg.header

@@ -432,7 +432,8 @@ adjust_outofscope_cursor_vars(struct cursor *cur)
 /* This tests whether the cursor was declared and opened in the same function. */
 #define SAMEFUNC(cur)	\
 	((cur->function == NULL) ||		\
-	 (cur->function != NULL && strcmp(cur->function, current_function) == 0))
+	 (cur->function != NULL && current_function != NULL && \
+	  strcmp(cur->function, current_function) == 0))
 
 static struct cursor *
 add_additional_variables(const char *name, bool insert)

src/interfaces/ecpg/preproc/ecpg.trailer

@@ -433,6 +433,7 @@ ECPGCursorStmt: DECLARE cursor_name cursor_options CURSOR opt_hold FOR prepared_
 		this->name = mm_strdup(@2);
 		this->function = (current_function ? mm_strdup(current_function) : NULL);
 		this->connection = connection ? mm_strdup(connection) : NULL;
+		this->opened = false;
 		this->command = mm_strdup(cat_str(6, "declare", cursor_marker, @3, "cursor", @5, "for $1"));
 		this->argsresult = NULL;
 		this->argsresult_oos = NULL;

src/interfaces/ecpg/preproc/variable.c

@@ -216,6 +216,9 @@ find_variable(const char *name)
 					case ']':
 						count--;
 						break;
+					case '\0':
+						mmfatal(PARSE_ERROR, "unmatched brace in variable \"%s\"", name);
+						break;
 					default:
 						break;
 				}