Allocate more space if necessary in RandSeedAddPerfMon

Currently we use a fixed buffer of 250000 bytes to request
HKEY_PERFORMANCE_DATA. In many cases this is not enough, causing the
entropy collection to be skipped.

Use a loop that grows the buffer as specified in the RegQueryValueEx
documentation:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms724911%28v=vs.85%29.aspx

(as the size of the performance data can differ for every call, the
normal solution of requesting the size then allocating that can't work)

src/util.cpp

@@ -169,8 +169,17 @@ void RandAddSeedPerfmon()
     // Don't need this on Linux, OpenSSL automatically uses /dev/urandom
     // Seed with the entire set of perfmon data
     std::vector <unsigned char> vData(250000,0);
-    unsigned long nSize = vData.size();
+    long ret = 0;
-    long ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", NULL, NULL, begin_ptr(vData), &nSize);
+    unsigned long nSize = 0;
+    const size_t nMaxSize = 10000000; // Bail out at more than 10MB of performance data
+    while (true)
+    {
+        nSize = vData.size();
+        ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", NULL, NULL, begin_ptr(vData), &nSize);
+        if (ret != ERROR_MORE_DATA || vData.size() >= nMaxSize)
+            break;
+        vData.resize(std::max((vData.size()*3)/2, nMaxSize)); // Grow size of buffer exponentially
+    }
     RegCloseKey(HKEY_PERFORMANCE_DATA);
     if (ret == ERROR_SUCCESS)
     {