Bug 1219310 - part 2 - keep track of how much pref file we have read; r=njn

Looking at a preference file read with strace typically looks like:

open("...", O_RDONLY) = X
...
read(X, "...", SIZE) = SIZE
read(X, "...", SIZE) = 0
...

There's no reason to call Read() and make another syscall to determine
there's no data left for reading.  We can keep track of how much we've
read at minimal cost and thus determine for ourselves when we are done.

modules/libpref/Preferences.cpp

@@ -1017,6 +1017,7 @@ static nsresult openPrefFile(nsIFile* aFile)
   // Read is not guaranteed to return a buf the size of fileSize,
   // but usually will.
   nsresult rv2 = NS_OK;
+  uint32_t offset = 0;
   for (;;) {
     uint32_t amtRead = 0;
     rv = inStr->Read((char*)fileBuffer, fileSize, &amtRead);
@@ -1024,6 +1025,10 @@ static nsresult openPrefFile(nsIFile* aFile)
       break;
     if (!PREF_ParseBuf(&ps, fileBuffer, amtRead))
       rv2 = NS_ERROR_FILE_CORRUPTED;
+    offset += amtRead;
+    if (offset == fileSize) {
+      break;
+    }
   }
 
   PREF_FinalizeParseState(&ps);