Fix compiler warnings on MSYS2

The PS_USE_NONE case in ps_status.c left a couple of unused variables
exposed.

Discussion: https://www.postgresql.org/message-id/flat/6b467edc-4018-521f-ab18-171f098557ca%402ndquadrant.com

src/backend/utils/misc/ps_status.c

@@ -89,6 +89,8 @@ bool		update_process_title = true;
 #endif
 
 
+#ifndef PS_USE_NONE
+
 #ifndef PS_USE_CLOBBER_ARGV
 /* all but one option need a buffer to write their ps line in */
 #define PS_BUFFER_SIZE 256
@@ -104,6 +106,8 @@ static size_t ps_buffer_cur_len;	/* nominal strlen(ps_buffer) */
 
 static size_t ps_buffer_fixed_size; /* size of the constant prefix */
 
+#endif							/* not PS_USE_NONE */
+
 /* save the original argv[] location here */
 static int	save_argc;
 static char **save_argv;
@@ -420,7 +424,11 @@ get_ps_display(int *displen)
 	}
 #endif
 
+#ifndef PS_USE_NONE
 	*displen = (int) (ps_buffer_cur_len - ps_buffer_fixed_size);
 
 	return ps_buffer + ps_buffer_fixed_size;
+#else
+	return "";
+#endif
 }