Remove unnecessary allowNull argument from getSocketAddress. NFC

The only callsite that was using this third argument was simply doing
and early return, we can just most the check to the caller and
simpify the code.

Author: sbc100

src/library_syscall.js

@@ -324,11 +324,8 @@ var SyscallsLibrary = {
 #endif
     return socket;
   },
-  /** @param {boolean=} allowNull */
   $getSocketAddress__deps: ['$readSockaddr', '$FS', '$DNS'],
-  $getSocketAddress__docs: '/** @param {boolean=} allowNull */',
-  $getSocketAddress: (addrp, addrlen, allowNull) => {
-    if (allowNull && addrp === 0) return null;
+  $getSocketAddress: (addrp, addrlen) => {
     var info = readSockaddr(addrp, addrlen);
     if (info.errno) throw new FS.ErrnoError(info.errno);
     info.addr = DNS.lookup_addr(info.addr) || info.addr;
@@ -421,11 +418,11 @@ var SyscallsLibrary = {
   __syscall_sendto__deps: ['$getSocketFromFD', '$getSocketAddress'],
   __syscall_sendto: (fd, message, length, flags, addr, addr_len) => {
     var sock = getSocketFromFD(fd);
-    var dest = getSocketAddress(addr, addr_len, true);
-    if (!dest) {
+    if (!addr) {
       // send, no address provided
       return FS.write(sock.stream, HEAP8, message, length);
     }
+    var dest = getSocketAddress(addr, addr_len);
     // sendto an address
     return sock.sock_ops.sendmsg(sock, HEAP8, message, length, dest.addr, dest.port);
   },