[RF] Fix wrong usage of std::ostringstream in RooProdPdf

After getting confused for some time why some fits don't work with the
new CPU backend, I found one of the problems: the `std::ostringstream`
was used incorrectly since 916a180 (commit from 10 years ago).

The `std::ostringstream` constructor arguments do not initialize the
stream with that string — the argument sets the open mode, not the
contents.

This can be seen with this little example in the interpreter.

```txt
root [0] const char *pfx = "hello";
root [1] std::ostringstream os(pfx);
root [2] os << " world";
root [3] os.str()
(std::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >::__string_type) " world"
root [4] .q
```

The result is that the prefix doesn't get added, and hence it can happen
that many nodes in the computation graph can have the same name,
breaking the assumptions of the new RooFit evaluation backend.

Author: guitargeek

roofit/roofitcore/src/RooProdPdf.cxx

@@ -1438,7 +1438,8 @@ std::string RooProdPdf::makeRGPPName(const char* pfx, const RooArgSet& term, con
 {
   // Make an appropriate automatic name for a RooGenProdProj object in getPartIntList()
 
-  std::ostringstream os(pfx);
+  std::ostringstream os;
+  os << pfx;
   os << "[";
 
   // Encode component names