patch 8.2.3116: Vim9: crash when debugging a function with line continuation

Problem:    Vim9: crash when debugging a function with line continuation.
Solution:   Check for a NULL pointer. (closes #8521)

src/testdir/test_debugger.vim

@@ -1009,6 +1009,7 @@ func Test_debug_def_function()
            eval 1
          enddef
     enddef
+
     def g:FuncComment()
       # comment
       echo "first"
@@ -1016,6 +1017,7 @@ func Test_debug_def_function()
       # comment
       echo "second"
     enddef
+
     def g:FuncForLoop()
       eval 1
       for i in [11, 22, 33]
@@ -1023,6 +1025,11 @@ func Test_debug_def_function()
       endfor
       echo "done"
     enddef
+
+    def g:FuncWithSplitLine()
+        eval 1
+           | eval 2
+    enddef
   END
   call writefile(file, 'Xtest.vim')
 
@@ -1078,6 +1085,12 @@ func Test_debug_def_function()
   call RunDbgCmd(buf, 'next', ['function FuncForLoop', 'line 2: for i in [11, 22, 33]'])
   call RunDbgCmd(buf, 'echo i', ['22'])
 
+  call RunDbgCmd(buf, 'breakdel *')
+  call RunDbgCmd(buf, 'cont')
+
+  call RunDbgCmd(buf, ':breakadd func FuncWithSplitLine')
+  call RunDbgCmd(buf, ':call FuncWithSplitLine()', ['function FuncWithSplitLine', 'line 1: eval 1 | eval 2'])
+
   call RunDbgCmd(buf, 'cont')
   call StopVimInTerminal(buf)
   call delete('Xtest.vim')

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3116,
 /**/
     3115,
 /**/

src/vim9execute.c

@@ -1497,9 +1497,11 @@ handle_debug(isn_T *iptr, ectx_T *ectx)
 	ga_init2(&ga, sizeof(char_u *), 10);
 	for (lnum = iptr->isn_lnum; lnum < end_lnum; ++lnum)
 	{
-	    char_u *p = skipwhite(
+	    char_u *p = ((char_u **)ufunc->uf_lines.ga_data)[lnum - 1];
-			       ((char_u **)ufunc->uf_lines.ga_data)[lnum - 1]);
 
+	    if (p == NULL)
+		continue;  // left over from continuation line
+	    p = skipwhite(p);
 	    if (*p == '#')
 		break;
 	    if (ga_grow(&ga, 1) == OK)