c-family: Fix option check in handle_pragma_diagnostic [PR106252]

In r13-1544, handle_pragma_diagnostic was refactored to support processing
early pragmas. During that process the part looking up option arguments was
inadvertenly moved too early, prior to checking the option was valid, causing
PR106252. Fixed by moving the check back where it goes.

gcc/c-family/ChangeLog:

	PR preprocessor/106252
	* c-pragma.cc (handle_pragma_diagnostic_impl): Don't look up the
	option argument prior to verifying the option was found.

diff --git a/gcc/c-family/c-pragma.cc b/gcc/c-family/c-pragma.cc
index 62bce2ed0f5..789719e6e6a 100644
--- a/gcc/c-family/c-pragma.cc
+++ b/gcc/c-family/c-pragma.cc
@@ -1009,10 +1009,6 @@ handle_pragma_diagnostic_impl ()
   if (early && !c_option_is_from_cpp_diagnostics (option_index))
     return;
 
-  const char *arg = NULL;
-  if (cl_options[option_index].flags & CL_JOINED)
-    arg = data.option_str + 1 + cl_options[option_index].opt_len;
-
   if (option_index == OPT_SPECIAL_unknown)
     {
       if (want_diagnostics)
@@ -1052,6 +1048,10 @@ handle_pragma_diagnostic_impl ()
       return;
     }
 
+  const char *arg = NULL;
+  if (cl_options[option_index].flags & CL_JOINED)
+    arg = data.option_str + 1 + cl_options[option_index].opt_len;
+
   struct cl_option_handlers handlers;
   set_default_handlers (&handlers, NULL);
   /* FIXME: input_location isn't the best location here, but it is