coverity/13750: Negative array index read: FP.

Problem    : Negative array index read @ 909.
Diagnostic : False positive.
Rationale  : Suggested error path assigns a negative value to idx at
             line 836 (`idx = find_command(ca.cmdchar);`). That's
             impossible, as `ca.cmdchar` is set to Ctrl_BSL just two
             lines above, so we know that value will be in the table.
Resolution : Assert idx >= 0.

src/nvim/normal.c

@@ -834,6 +834,7 @@ getcount:
             ca.cmdchar = Ctrl_BSL;
             ca.nchar = c;
             idx = find_command(ca.cmdchar);
+            assert(idx >= 0);
           }
         }
       }