fix array index out of bounds

[danielmarjamaki]

Summary

This little patch fixes possible array index out of bounds:

[tools/clang/lib/Driver/MSVCToolChain.cpp:147]: (error) Array 'partialKey[256]' accessed at index 256, which is out of bounds

There is a slight change in behaviour now. If partialKey is filled with data then partialData[255] is zeroed instead of partialData[256]. I thought about making partialData one byte bigger so partialData[256]='\0' would be ok but I _think_ that it is 256 elements by intention.

[danielmarjamaki]

adding reviewers according to 'svn blame' and CODE_OWNERS.txt

[rnk]

lgtm

This code is old and super janky. =/

cfe/trunk/lib/Driver/MSVCToolChain.cpp

if (placeHolder) {
     // Find end of previous key.
     while ((keyEnd > keyPath) && (*keyEnd != '\\'))
       keyEnd--;
     // Find end of key containing $VERSION.
     while (*nextKey && (*nextKey != '\\'))
       nextKey++;
     size_t partialKeyLength = keyEnd - keyPath;
     char partialKey[256];
-    if (partialKeyLength > sizeof(partialKey))
-      partialKeyLength = sizeof(partialKey);
+    if (partialKeyLength >= sizeof(partialKey))
+      partialKeyLength = sizeof(partialKey) - 1;
     strncpy(partialKey, keyPath, partialKeyLength);
     partialKey[partialKeyLength] = '\0';
     HKEY hTopKey = NULL;
     lResult = RegOpenKeyEx(hRootKey, partialKey, 0, KEY_READ | KEY_WOW64_32KEY,
                            &hTopKey);
     if (lResult == ERROR_SUCCESS) {
       char keyName[256];
       double bestValue = 0.0;