The previous patch not only moved code, it also changed the check.
Negative gain values seem to be valid after all, there just was the
"magic" value 0.0 which means "not available".  This patch changes the
"magic" value to "INFINITY", and uses the C99 function isinf() to
check.  It might have been a better idea to use "NAN", but the "NAN"
macro is a GNU extension.