util/FormatString: pass the allocated buffer to AllocatedString::Donate()

.. and not the stack buffer.  This made the AllocatedString destructor
crash.

Closes #52

src/util/FormatString.cxx

@@ -57,7 +57,7 @@ FormatStringV(const char *fmt, va_list args)
 	const size_t length = strlen(buffer);
 	char *p = new char[length + 1];
 	memcpy(p, buffer, length + 1);
-	return AllocatedString<>::Donate(buffer);
+	return AllocatedString<>::Donate(p);
 #endif
 }