Fixed ‘length’ may be used uninitialized in Array.prototype.pop().

Fixed ‘length’ may be used uninitialized in Array.prototype.pop().

When building by GCC with -O3 and -flto flags the following
warning was reported:
src/njs_array.c: In function ‘njs_array_prototype_pop’:
src/njs_array.c:1009:8: error: ‘length’ may be used uninitialized in
this function [-Werror=maybe-uninitialized]
1009 | if (length == 0) {
     |     ^

Returning a specific code in njs_value_to_number() helps GCC
to infer that there are only 2 return values are possible and
both of them are handled.