patch 8.1.0802: negative index doesn't work for Blob

Problem:    Negative index doesn't work for Blob.
Solution:   Make it work, add a test. (closes #3856)
diff --git a/src/blob.c b/src/blob.c
index a954030..9dc7926 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -72,8 +72,12 @@
 	int  len = from->vval.v_blob->bv_ga.ga_len;
 
 	if (len > 0)
+	{
 	    to->vval.v_blob->bv_ga.ga_data =
 			    vim_memsave(from->vval.v_blob->bv_ga.ga_data, len);
+	    if (to->vval.v_blob->bv_ga.ga_data == NULL)
+		len = 0;
+	}
 	to->vval.v_blob->bv_ga.ga_len = len;
     }
     return ret;
@@ -112,7 +116,7 @@
  * Get byte "idx" in blob "b".
  * Caller must check that "idx" is valid.
  */
-    char_u
+    int
 blob_get(blob_T *b, int idx)
 {
     return ((char_u*)b->bv_ga.ga_data)[idx];
diff --git a/src/eval.c b/src/eval.c
index dac086f..f111dc4 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4723,12 +4723,13 @@
 		}
 		else
 		{
-		    // The resulting variable is a string of a single
-		    // character.  If the index is too big or negative the
-		    // result is empty.
+		    // The resulting variable is a byte value.
+		    // If the index is too big or negative that is an error.
+		    if (n1 < 0)
+			n1 = len + n1;
 		    if (n1 < len && n1 >= 0)
 		    {
-			int v = (int)blob_get(rettv->vval.v_blob, n1);
+			int v = blob_get(rettv->vval.v_blob, n1);
 
 			clear_tv(rettv);
 			rettv->v_type = VAR_NUMBER;
diff --git a/src/proto/blob.pro b/src/proto/blob.pro
index b8e48de..019692d 100644
--- a/src/proto/blob.pro
+++ b/src/proto/blob.pro
@@ -6,7 +6,7 @@
 void blob_free(blob_T *b);
 void blob_unref(blob_T *b);
 long blob_len(blob_T *b);
-char_u blob_get(blob_T *b, int idx);
+int blob_get(blob_T *b, int idx);
 void blob_set(blob_T *b, int idx, char_u c);
 int blob_equal(blob_T *b1, blob_T *b2);
 int read_blob(FILE *fd, blob_T *blob);
diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 9eb2057..b01e11b 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -95,6 +95,13 @@
   call assert_equal(999, get(b, 5, 999))
   call assert_equal(-1, get(b, -8))
   call assert_equal(999, get(b, -8, 999))
+
+  call assert_equal(0x00, b[0])
+  call assert_equal(0x22, b[2])
+  call assert_equal(0x44, b[4])
+  call assert_equal(0x44, b[-1])
+  call assert_fails('echo b[5]', 'E979:')
+  call assert_fails('echo b[-8]', 'E979:')
 endfunc
 
 func Test_blob_to_string()
diff --git a/src/version.c b/src/version.c
index d662965..5b404ed 100644
--- a/src/version.c
+++ b/src/version.c
@@ -792,6 +792,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    802,
+/**/
     801,
 /**/
     800,