Removed misused __attribute__((malloc)).

Removed misused __attribute__((malloc)).

According to the documentation:

| This tells the compiler that a function is malloc-like, i.e., that the
| pointer P returned by the function cannot alias any other pointer valid
| when the function returns, and moreover no pointers to valid objects
| occur in any storage addressed by P.

The njs_string_alloc() allocates a storage for a string and makes the
storage accessible via the passed "value" argument.  The function also
returns an intermediate pointer which is used only for string content
initialization and then discarded.  Since the pointer is not stored
anywhere after the initialization, GCC with -O or higher optimisation
levels rightfully optimizes out the initialization.