Return false when initial check in convert_from_string fails

Otherwise, we currently have undefined behavior, as this method returns a bool, so we cannot just return without specifying what we are returning.

include/boost/math/cstdfloat/cstdfloat_iostream.hpp

@@ -571,7 +571,7 @@
 
     if((p == static_cast<const char*>(0U)) || (*p == static_cast<char>(0)))
     {
-      return;
+      return false;
     }
 
     bool is_neg       = false;