[5.5][RemoteMirror] Add NULL checks to Remote Mirror malloc calls.

These calls can fail when passed absurd sizes, which can happen when we try to read data that's corrupt or doesn't contain what we think it should. Fail gracefully instead of crashing.

rdar://78210820

Author: mikeash

include/swift/Remote/MemoryReader.h

@@ -100,6 +100,8 @@ class MemoryReader {
   virtual ReadBytesResult
   readBytes(RemoteAddress address, uint64_t size) {
     auto *Buf = malloc(size);
+    if (!Buf)
+      return ReadBytesResult{};
     ReadBytesResult Result(Buf, [](const void *ptr) {
       free(const_cast<void *>(ptr));
     });

include/swift/Remote/MetadataReader.h

@@ -1802,6 +1802,9 @@ class MetadataReader {
   MetadataRef _readMetadata(StoredPointer address, size_t sizeAfter) {
     auto size = sizeAfter;
     uint8_t *buffer = (uint8_t *) malloc(size);
+    if (!buffer)
+      return nullptr;
+
     if (!Reader->readBytes(RemoteAddress(address), buffer, size)) {
       free(buffer);
       return nullptr;
@@ -2522,6 +2525,8 @@ class MetadataReader {
   std::string readObjCProtocolName(StoredPointer Address) {
     auto Size = sizeof(TargetObjCProtocolPrefix<Runtime>);
     auto Buffer = (uint8_t *)malloc(Size);
+    if (!Buffer)
+      return std::string();
     SWIFT_DEFER {
       free(Buffer);
     };