sqlite3: fix internal return type violation in escapeString()

If this call fails due to an internal libsqlite3 error, then the
function will return NULL (as that's the default value set by the VM).
However, the function is marked with a non-nullable string return type.
Therefore this will result in a type violation and a fatal error in
debug mode.
Either we solve it by making the function nullable or throw.
I chose the latter as it is less of a footgun.

Author: ndossche

ext/sqlite3/sqlite3.c

@@ -494,6 +494,9 @@ PHP_METHOD(SQLite3, escapeString)
 		if (ret) {
 			RETVAL_STRING(ret);
 			sqlite3_free(ret);
+		} else {
+			zend_throw_exception_ex(php_sqlite3_exception_ce, 0, "Unable to escape string");
+			RETURN_THROWS();
 		}
 	} else {
 		RETURN_EMPTY_STRING();