regcomp.c: Handle /ss\xdf/iaa properly

Having both ss and \xdf in a string caused the node type to be changed
back to a wrong one.

This fixes #17486

regcomp.c

@@ -4440,7 +4440,7 @@ S_join_exact(pTHX_ RExC_state_t *pRExC_state, regnode *scan,
                 s++;
             }
         }
-	else {
+	else if (OP(scan) != EXACTFAA_NO_TRIE) {
 
             /* Non-UTF-8 pattern, not EXACTFAA node.  Look for the multi-char
              * folds that are all Latin1.  As explained in the comments

t/re/pat_advanced.t

@@ -2527,6 +2527,11 @@ EOF
                       "", {}, "*COMMIT caused positioning beyond EOS");
     }
 
+    {   # [GH #17486]    Previously assertion failure
+        fresh_perl_is('0=~/(?iaa)ss\337(?0)|/',
+                      "", {}, "EXACTFUP node isn't changed into something else");
+    }
+
 
     # !!! NOTE that tests that aren't at all likely to crash perl should go
     # a ways above, above these last ones.  There's a comment there that, like
@@ -2536,3 +2541,4 @@ EOF
 } # End of sub run_tests
 
 1;
+0=~/(?iaa)ss\337(?0)|/