patch 8.2.4440: crash with specific regexp pattern and string

Problem:    Crash with specific regexp pattern and string.
Solution:   Stop at the start of the string.

src/regexp_bt.c

@@ -4615,6 +4615,11 @@ regmatch(
 			    if (rex.input == rex.line)
 			    {
 				// backup to last char of previous line
+				if (rex.lnum == 0)
+				{
+				    status = RA_NOMATCH;
+				    break;
+				}
 				--rex.lnum;
 				rex.line = reg_getline(rex.lnum);
 				// Just in case regrepeat() didn't count

src/testdir/test_regexp_utf8.vim

@@ -508,7 +508,6 @@ endfunc
 " Check that [[:upper:]] matches for automatic engine
 func Test_match_char_class_upper()
   new
-  let _engine=&regexpengine
 
   " Test 1: [[:upper:]]\{2,\}
   set regexpengine=0
@@ -549,7 +548,7 @@ func Test_match_char_class_upper()
   call assert_equal(4, searchcount().total, 'TEST 3 lower')
 
   " clean up
-  let &regexpengine=_engine
+  set regexpengine=0
   bwipe!
 endfunc
 
@@ -561,4 +560,13 @@ func Test_match_invalid_byte()
   call delete('Xinvalid')
 endfunc
 
+func Test_match_too_complicated()
+  set regexpengine=1
+  exe "vsplit \xeb\xdb\x99"
+  silent! buf \&\zs*\zs*0
+  bwipe!
+  set regexpengine=0
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4440,
 /**/
     4439,
 /**/