Avoid overallocating for strings (#1099)

Author: DmitriyMusatkin

source/string.c

@@ -189,11 +189,8 @@ struct aws_string *aws_string_new_from_c_str(struct aws_allocator *allocator, co
 struct aws_string *aws_string_new_from_array(struct aws_allocator *allocator, const uint8_t *bytes, size_t len) {
     AWS_PRECONDITION(allocator);
     AWS_PRECONDITION(AWS_MEM_IS_READABLE(bytes, len));
-    size_t malloc_size;
-    if (aws_add_size_checked(sizeof(struct aws_string) + 1, len, &malloc_size)) {
-        return NULL;
-    }
-    struct aws_string *str = aws_mem_acquire(allocator, malloc_size);
+
+    struct aws_string *str = aws_mem_acquire(allocator, offsetof(struct aws_string, bytes[len + 1]));
     if (!str) {
         return NULL;
     }