LibC: Some calloc() and realloc() improvements (#3108)

If the space cannot be allocated, the original memory block shall remain
unchanged and the function should return nullptr.

Also add a function attribute and some null checks.

Libraries/LibC/stdlib.h

@@ -42,7 +42,7 @@ __attribute__((malloc)) __attribute__((alloc_size(1))) void* malloc(size_t);
 __attribute__((malloc)) __attribute__((alloc_size(1, 2))) void* calloc(size_t nmemb, size_t);
 size_t malloc_size(void*);
 void free(void*);
-void* realloc(void* ptr, size_t);
+__attribute__((alloc_size(2))) void* realloc(void* ptr, size_t);
 char* getenv(const char* name);
 int putenv(char*);
 int unsetenv(const char*);