patch 8.1.0968: crash when using search pattern \%Ufffffc23

Problem:    Crash when using search pattern \%Ufffffc23.
Solution:   Limit character to INT_MAX. (closes #4009)

src/regexp_nfa.c

@@ -1475,7 +1475,7 @@ nfa_regatom(void)
 			    default:  nr = -1; break;
 			}
 
-			if (nr < 0)
+			if (nr < 0 || nr > INT_MAX)
 			    EMSG2_RET_FAIL(
 			       _("E678: Invalid character after %s%%[dxouU]"),
 				    reg_magic == MAGIC_ALL);

src/testdir/test_search.vim

@@ -1211,3 +1211,12 @@ func Test_search_Ctrl_L_combining()
   call assert_equal(bufcontent[1], @/)
   call Incsearch_cleanup()
 endfunc
+
+func Test_large_hex_chars()
+  " This used to cause a crash, the character becomes an NFA state.
+  try
+    /\%Ufffffc23
+  catch
+    call assert_match('E678:', v:exception)
+  endtry
+endfunc

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    968,
 /**/
     967,
 /**/