Don't realloc(nullptr) from PSRAM by default

Several Arduino APIs realloc(NULL) which is legal and equivalent to
"malloc()", but the PSRAM logic was placing those malloc calls in PSRAM
and not RAM because "0" < RAM_START.

Ensure the realloc address is non-null and before RAM_START before
using PSRAM.

Author: earlephilhower

cores/rp2040/malloc-lock.cpp

@@ -88,7 +88,7 @@ extern "C" void *__wrap_realloc(void *mem, size_t size) {
     void *rc;
     noInterrupts();
 #ifdef RP2350_PSRAM_CS
-    if (mem < __ram_start) {
+    if (mem && (mem < __ram_start)) {
         rc = __psram_realloc(mem, size);
     } else {
         rc = __real_realloc(mem, size);
@@ -103,7 +103,7 @@ extern "C" void *__wrap_realloc(void *mem, size_t size) {
 extern "C" void __wrap_free(void *mem) {
     noInterrupts();
 #ifdef RP2350_PSRAM_CS
-    if (mem < __ram_start) {
+    if (mem && (mem < __ram_start)) {
         __psram_free(mem);
     } else {
         __real_free(mem);