Fix build with current GCC: EL_EXPLICIT(location) always non-NULL

Fix build with current GCC: EL_EXPLICIT(location) always non-NULL

Compiling GDB with current GCC (1b4a63593b) runs into this:

  src/gdb/location.c: In function 'int event_location_empty_p(const event_location*)':
  src/gdb/location.c:963:38: error: the address of 'event_location::<unnamed union>::explicit_loc' will never be NULL [-Werror=address]
    963 |       return (EL_EXPLICIT (location) == NULL
	|                                      ^
  src/gdb/location.c:57:30: note: 'event_location::<unnamed union>::explicit_loc' declared here
     57 |     struct explicit_location explicit_loc;
	|                              ^~~~~~~~~~~~

GCC is right, EL_EXPLICIT is defined as returning the address of an
union field:

      /* An explicit location.  */
      struct explicit_location explicit_loc;
  #define EL_EXPLICIT(P) (&((P)->u.explicit_loc))

and thus must always be non-NULL.

Change-Id: Ie74fee7834495a93affcefce03c06e4d83ad8191