Fix minor issue in BString I noticed when researching #7226:

* if _MakeWritable() fails, LockBuffer() must return NULL, not what
  happens to be in fPrivateData

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40525 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kits/support/String.cpp

@@ -1787,10 +1787,12 @@ BString::LockBuffer(int32 maxLength)
 	if (maxLength > length)
 		length = maxLength;
 
-	if (_MakeWritable(length, true) == B_OK) {
+	if (_MakeWritable(length, true) != B_OK)
-		_ReferenceCount() = -1;
+		return NULL;
-			// mark unshareable
+
-	}
+	_ReferenceCount() = -1;
+		// mark unshareable
+
 	return fPrivateData;
 }