Remove long_u: do_outofmem_msg().

Remove long_u occurrences due to do_outofmem_msg() function.
Refactor size parameter from long_u into size_t.

src/nvim/memory.c

@@ -84,7 +84,7 @@ void *verbose_try_malloc(size_t size)
 {
   void *ret = try_malloc(size);
   if (!ret) {
-    do_outofmem_msg((long_u)size);
+    do_outofmem_msg(size);
   }
   return ret;
 }
@@ -214,7 +214,7 @@ void *xmemdup(const void *data, size_t len)
  * Avoid repeating the error message many times (they take 1 second each).
  * Did_outofmem_msg is reset when a character is read.
  */
-void do_outofmem_msg(long_u size)
+void do_outofmem_msg(size_t size)
 {
   if (!did_outofmem_msg) {
     /* Don't hide this message */

src/nvim/memory.h

@@ -134,7 +134,7 @@ char *xstpncpy(char *restrict dst, const char *restrict src, size_t maxlen);
 void *xmemdup(const void *data, size_t len)
  FUNC_ATTR_MALLOC FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_RET;
 
-void do_outofmem_msg(long_u size);
+void do_outofmem_msg(size_t size);
 void free_all_mem(void);
 
 #endif

src/nvim/screen.c

@@ -6288,7 +6288,7 @@ retry:
       || outofmem) {
     if (ScreenLines != NULL || !done_outofmem_msg) {
       /* guess the size */
-      do_outofmem_msg((long_u)((Rows + 1) * Columns));
+      do_outofmem_msg((Rows + 1) * Columns);
 
       /* Remember we did this to avoid getting outofmem messages over
        * and over again. */