ga_growsize should be >= 1

I know it could be 0 sometimes. Running the tests with
`assert(gap->ga_growsize > 0)` in ga_grow() crashes nvim while running the
tests.

 - Add a setter for ga_growsize that checks whether the value passed is >=1 (log
	 in case it's not)
 - log when ga_grow() tries to use a ga_growsize that's not >=1
 - use GA_EMPTY_INIT_VALUE is many places

src/nvim/menu.c

@@ -1434,7 +1434,7 @@ typedef struct {
   char_u      *to;              /* translated name */
 } menutrans_T;
 
-static garray_T menutrans_ga = {0, 0, 0, 0, NULL};
+static garray_T menutrans_ga = GA_EMPTY_INIT_VALUE;
 
 /*
  * ":menutrans".