Fixes random crashes while on the PicoW with WiFi.

The system malloc/etc. is not re-entrant, so it is not safe to do
any memory (de)allocation from an interrupt.

The LWIP stack is configured not to use the memory manager, but it may
end up calling callback functions in userspace which may well use
new or malloc() (i.e. to manage a new connection info or process a
UDP packet like in MDNS).

This could cause unpredicatable crashes due to memory corruption at
some random time after the interrupt/malloc occurred.

Avoid the issue completely by locking interrupts out while in the memory
manager.  Inter-core will be protected by the malloc_lock already in
Newlib.

This should not have any effect on absolute performance of the system.

Brute-force protect all LWIP calls from re-rentrancy

Was causing memory corruption during long, high frequency, multi-client
workloads.  It is illegal to call netif->input() from an IRQ while
in LWIP code.

For now, throw them out and let upper layer handle a re-transmit.

During a ClientContext we set TCP connection arguments to nullptr and then
turn off all the callbacks.  It is possible for an interrupt to occur
between setting the arg nullptr and the clearing of all CBs.  The the CB
function will get nullptr as (this), causing a crash like:
````
Thread 1 received signal SIGTRAP, Trace/breakpoint trap.
isr_hardfault () at /home/earle/Arduino/hardware/pico/rp2040/pico-sdk/src/rp2_common/pico_standard_link/crt0.S:98
98	decl_isr_bkpt isr_hardfault
(gdb) where
   e/Arduino/hardware/pico/rp2040/libraries/WiFi/src/include/ClientContext.h:619
   ries/WiFi/src/include/ClientContext.h:612
   raries/WiFi/src/include/ClientContext.h:671
    ) at /home/earle/Arduino/hardware/pico/rp2040/pico-sdk/lib/lwip/src/core/tcp_in.c:541
    ) at /home/earle/Arduino/hardware/pico/rp2040/pico-sdk/lib/lwip/src/core/ipv4/ip4.c:743
   rdware/pico/rp2040/pico-sdk/lib/lwip/src/netif/ethernet.c:186
    buf=0x20001b7a <cyw43_state+226> "(\315\301") at /home/earle/Arduino/hardware/pico/rp2040/libraries/lwIP_CYW43/src/utility/CYW43shim.cpp:132
   2040/pico-sdk/lib/cyw43-driver/src/cyw43_ll.c:1212
   cores/rp2040/lwip_wrap.cpp:163
   e/ClientContext.h:85
   /libraries/WiFi/src/WiFiClient.cpp:81
   /libraries/WiFi/src/WiFiClient.cpp:78
   rver.cpp:284
   ico/rp2040/libraries/WebServer/src/WebServerTemplate.h:86
````

Avoid by checking and silently ignoring CBs who have a nullptr arg.