Memory functions: Improve style.

`try_malloc` was changed in 8bb2c2c0742c57150655e18cf8418a758cebdce8 to
avoid a warning when size is 0.

Code style was also improved on suggestion by @aktau.
This just applies the same style improvements to other similar
functions, to keep code homogeneous.
On November 24, 2014 11:35:15 AM EAT, "Eliseo Martínez" <notifications@github.com> wrote:
>> @@ -102,10 +102,12 @@ void *xmalloc(size_t size)
>>  void *xcalloc(size_t count, size_t size)
>>    FUNC_ATTR_MALLOC FUNC_ATTR_ALLOC_SIZE_PROD(1, 2)
>FUNC_ATTR_NONNULL_RET
>>  {
>> -  void *ret = count && size ? calloc(count, size) : calloc(1, 1);
>> +  size_t real_count = count && size ? count : 1;
>> +  size_t real_size = count && size ? size : 1;
>
>My first option here was `effective_size` / `effective_count`, but it
>was a bit long, so I opted for `real_size` / `real_count`.
>`allocated_size` / `allocated_count` was another option.

"Allocated" variant was the first thing I thought about when saw previous message. _or_1 looks rather ugly.

>
>To me, any of those are better because they introduce a clear concept.
>There is a "requested" size and a "real / effective / allocated" size.
>The fact that difference is being 1 or something else is secondary to
>that concept. Under this considerations:
>```calloc(real_size, real_count)```
>reads better to me than
>```calloc(size_or_1, count_or_1)```
>
>But this kind of things are very subjective.
>I don't have problems to changing names to whatever agreed.
>Note that these are very short functions, perfectly understandable
>anyway.
>
>---
>Reply to this email directly or view it on GitHub:
>https://github.com/neovim/neovim/pull/1526/files#r20777338