Add gcc function attributes to MyRocks code with a sprinkle of assertions

Summary:
Annotate MyRocks to document the current contracts using function attributes. We don't have the full power of SAL (Standard Annotation Language), but this is still better than nothing. Both Clang and GCC understand the attributes and `include/my_attribute.h` has the magic to disable them for compilers which don't support those directives.

The most useful thing we get out of these changes are the annotations using `warn_unused_result` because that's the only thing which `gcc` can reasonably enforce.

Using `nonnull` will help compiler to only catch the basic cases (passing `nullptr` or `NULL` literally) and rest of the attributes have the main benefit of guiding the compiler to apply more complex optimizations. Enhanced error deduction will require running a dedicated static analysis tool on the codebase (`scan-build` at some point in future?).

Where applicable add more assertions to code to increase the amount of internal consistency checks. Fix a couple of small bugs found by `warn_unused_result` as well.

Test Plan:
Classics:

```
mysqlbuild.sh
mysqltest.sh
mysqltest.sh --testset=RocksDB

mysqlbuild.sh --release
mysqltest.sh --release
mysqltest.sh --release --testset=RocksDB
```

Reviewers: alexyang, jkedgar

Reviewed By: jkedgar

Subscribers: MarkCallaghan, webscalesql-eng

Differential Revision: https://reviews.facebook.net/D55521

Author: Gunnar Kudrjavets

include/my_sys.h

@@ -152,8 +152,10 @@ typedef struct my_aio_result {
 #define GETDATE_FIXEDLENGTH	16
 
 	/* defines when allocating data */
-extern void *my_malloc(size_t Size,myf MyFlags);
-extern void *my_multi_malloc(myf MyFlags, ...);
+extern void *my_malloc(size_t Size,myf MyFlags)
+  __attribute__((__malloc__, __warn_unused_result__));
+extern void *my_multi_malloc(myf MyFlags, ...)
+  __attribute__((__malloc__, __warn_unused_result__));
 extern void *my_realloc(void *oldpoint, size_t Size, myf MyFlags);
 extern void my_free(void *ptr);
 extern void *my_memdup(const void *from,size_t length,myf MyFlags);

sql/thr_malloc.h

@@ -22,8 +22,8 @@ typedef struct charset_info_st CHARSET_INFO;
 typedef struct st_mem_root MEM_ROOT;
 
 void init_sql_alloc(MEM_ROOT *root, uint block_size, uint pre_alloc_size);
-void *sql_alloc(size_t);
-void *sql_calloc(size_t);
+void *sql_alloc(size_t) __attribute__((__malloc__, __warn_unused_result__));
+void *sql_calloc(size_t) __attribute__((__malloc__, __warn_unused_result__));
 char *sql_strdup(const char *str);
 char *sql_strmake(const char *str, size_t len);
 void *sql_memdup(const void * ptr, size_t size);