patch 8.2.2483: Vim9: type error for misformed expression

Problem:    Vim9: type error for misformed expression.
Solution:   Check for end of command before checking type. (closes #7795)

Author: brammool

src/testdir/test_vim9_script.vim

@@ -1743,6 +1743,21 @@ def Test_if_elseif_else_fails()
   CheckDefFailure(['endif'], 'E580:')
   CheckDefFailure(['if g:abool', 'elseif xxx'], 'E1001:')
   CheckDefFailure(['if true', 'echo 1'], 'E171:')
+
+  var lines =<< trim END
+      var s = ''
+      if s = ''
+      endif
+  END
+  CheckDefFailure(lines, 'E488:')
+
+  lines =<< trim END
+      var s = ''
+      if s == ''
+      elseif s = ''
+      endif
+  END
+  CheckDefFailure(lines, 'E488:')
 enddef
 
 let g:bool_true = v:true
@@ -2200,6 +2215,13 @@ def Test_while_loop_fails()
   CheckDefFailure(['break'], 'E587:')
   CheckDefFailure(['if true', 'break'], 'E587:')
   CheckDefFailure(['while 1', 'echo 3'], 'E170:')
+
+  var lines =<< trim END
+      var s = ''
+      while s = ''
+      endwhile
+  END
+  CheckDefFailure(lines, 'E488:')
 enddef
 
 def Test_interrupt_loop()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2483,
 /**/
     2482,
 /**/

src/vim9compile.c

@@ -6701,6 +6701,11 @@ compile_if(char_u *arg, cctx_T *cctx)
 	clear_ppconst(&ppconst);
 	return NULL;
     }
+    if (!ends_excmd2(arg, skipwhite(p)))
+    {
+	semsg(_(e_trailing_arg), p);
+	return NULL;
+    }
     if (cctx->ctx_skip == SKIP_YES)
 	clear_ppconst(&ppconst);
     else if (instr->ga_len == instr_count && ppconst.pp_used == 1)
@@ -6825,6 +6830,11 @@ compile_elseif(char_u *arg, cctx_T *cctx)
 	return NULL;
     }
     cctx->ctx_skip = save_skip;
+    if (!ends_excmd2(arg, skipwhite(p)))
+    {
+	semsg(_(e_trailing_arg), p);
+	return NULL;
+    }
     if (scope->se_skip_save == SKIP_YES)
 	clear_ppconst(&ppconst);
     else if (instr->ga_len == instr_count && ppconst.pp_used == 1)
@@ -7237,6 +7247,11 @@ compile_while(char_u *arg, cctx_T *cctx)
     // compile "expr"
     if (compile_expr0(&p, cctx) == FAIL)
 	return NULL;
+    if (!ends_excmd2(arg, skipwhite(p)))
+    {
+	semsg(_(e_trailing_arg), p);
+	return NULL;
+    }
 
     if (bool_on_stack(cctx) == FAIL)
 	return FAIL;