vim-patch:8.0.0374: invalid memory access when using :sc in Ex mode (#7849)

Problem:    Invalid memory access when using :sc in Ex mode. (Dominique Pelle)
Solution:   Avoid the column being negative.  Also fix a hang in Ex mode.

ba748c8a84

src/nvim/ex_getln.c

@@ -2164,7 +2164,13 @@ getexmodeline (
     /* Get one character at a time.  Don't use inchar(), it can't handle
      * special characters. */
     prev_char = c1;
-    c1 = vgetc();
+
+    // Check for a ":normal" command and no more characters left.
+    if (ex_normal_busy > 0 && typebuf.tb_len == 0) {
+        c1 = '\n';
+    } else {
+        c1 = vgetc();
+    }
 
     /*
      * Handle line editing.