Utils/FileLoading: Fix LoadFileImpl

It is not an error that pread returns /less/ than what was requested. In
fact it's very common for the Linux kernel to return less than the data
requested from procfs.

procfs keeps coming back to bite this function, previously it was fstat
returning size of 0 which it hit. Now it only feeds data as much as it
wants per loop. In particular /proc/self/maps would only read ~3k bytes
on my system, but not be complete.

To fully fix the issue, always make sure to keep reading until there is
either an error OR zero is reached!

FEXCore/Source/Utils/FileLoading.cpp

@@ -31,24 +31,28 @@ static bool LoadFileImpl(T& Data, const fextl::string& Filepath, size_t FixedSiz
     FileSize = FixedSize;
   }
 
+  ssize_t CurrentOffset = 0;
   ssize_t Read = -1;
   bool LoadedFile {};
   if (FileSize) {
     // File size is known upfront
     Data.resize(FileSize);
-    Read = pread(FD, &Data.at(0), FileSize, 0);
+    while (CurrentOffset != FileSize && (Read = pread(FD, &Data.at(CurrentOffset), FileSize, 0)) > 0) {
+      CurrentOffset += Read;
+    }
 
-    LoadedFile = Read == FileSize;
+    LoadedFile = CurrentOffset == FileSize && Read != -1;
   } else {
     // The file is either empty or its size is unknown (e.g. procfs data).
     // Try reading in chunks instead
-    ssize_t CurrentOffset = 0;
     constexpr size_t READ_SIZE = 4096;
     Data.resize(READ_SIZE);
 
-    while ((Read = pread(FD, &Data.at(CurrentOffset), READ_SIZE, CurrentOffset)) == READ_SIZE) {
+    while ((Read = pread(FD, &Data.at(CurrentOffset), READ_SIZE, CurrentOffset)) > 0) {
       CurrentOffset += Read;
-      Data.resize(CurrentOffset + Read);
+      if ((CurrentOffset + READ_SIZE) > Data.size()) {
+        Data.resize(CurrentOffset + READ_SIZE);
+      }
     }
 
     if (Read == -1) {