Requesting external_pid_file with postgres -C when not initialized lead to coredump

Hi,

I faced a coredump when reading the value of the parameter
"external_pid_file" when it was not initialized in postgresql.conf
This came from the value not being specified to be initialized to en empty
string in guc.c in the ConfigureNamesString array.
the behavior can easily been tested with the following commands :
initdb test
postgres -D test -C external_pid_file

I faced the problem with version 9.3, 9.5 and 9.6 beta 1
This seems to come from a long time ago.

I wrote a patch ( with help from Stéphane Schildknecht ) to correct the
problem with a proper initialization.
The patch also removed a useless initialization of cluster_name to save a
little memory.

So, here is my first patch for PostgreSQL.

Best regards.

--
Alain Radix