patch 8.2.4914: string interpolation in :def function may fail

Problem:    String interpolation in :def function may fail.
Solution:   Do not terminate the expression. (closes #10377)

src/testdir/test_vim9_expr.vim

@@ -2143,6 +2143,19 @@ def Test_expr8_string()
   v9.CheckDefAndScriptFailure(['var x = "abc'], 'E114:', 1)
   v9.CheckDefAndScriptFailure(["var x = 'abc"], 'E115:', 1)
   v9.CheckDefFailure(["if 0", "echo 'xx", "endif"], 'E115', 2)
+
+  # interpolated string
+  var val = 'val'
+  var vv = $"some {val}"
+  assert_equal('some val', vv)
+  vv = $'other {val}'
+  assert_equal('other val', vv)
+
+  var x = 'x'
+  var vl = 'foo xxx bar xxx baz'
+              ->split($'x{x}x')
+              ->map((_, v: string) => v =~ 'bar')
+  assert_equal([false, true, false], vl)
 enddef
 
 def Test_expr8_vimvar()

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4914,
 /**/
     4913,
 /**/

src/vim9compile.c

@@ -981,7 +981,6 @@ compile_all_expr_in_str(char_u *str, int evalstr, cctx_T *cctx)
 {
     char_u	*p = str;
     char_u	*val;
-    char_u	save_c;
     int		count = 0;
 
     if (cctx->ctx_skip == SKIP_YES)
@@ -1051,11 +1050,8 @@ compile_all_expr_in_str(char_u *str, int evalstr, cctx_T *cctx)
 	    semsg(_(e_missing_close_curly_str), str);
 	    return FAIL;
 	}
-	save_c = *block_end;
-	*block_end = NUL;
 	if (compile_expr0(&block_start, cctx) == FAIL)
 	    return FAIL;
-	*block_end = save_c;
 	may_generate_2STRING(-1, TRUE, cctx);
 	++count;