patch 8.0.0879: crash when shifting with huge number

Problem:    Crash when shifting with huge number.
Solution:   Check for overflow. (Dominique Pelle, closes #1945)

src/ops.c

@@ -396,7 +396,10 @@ shift_block(oparg_T *oap, int amount)
 	return;
 
     /* total is number of screen columns to be inserted/removed */
-    total = amount * p_sw;
+    total = (int)((unsigned)amount * (unsigned)p_sw);
+    if ((total / p_sw) != amount)
+	return; /* multiplication overflow */
+
     oldp = ml_get_curline();
 
     if (!left)

src/testdir/test_visual.vim

@@ -18,6 +18,14 @@ func Test_block_shift_multibyte()
   q!
 endfunc
 
+func Test_block_shift_overflow()
+  " This used to cause a multiplication overflow followed by a crash.
+  new
+  normal ii
+  exe "normal \<C-V>876543210>"
+  q!
+endfunc
+
 func Test_dotregister_paste()
   new
   exe "norm! ihello world\<esc>"

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    879,
 /**/
     878,
 /**/