Bug in bessel_K

<div id="comment:0"></div>


```
sage: bessel_K(0,0)
...
NameError: global name 'infinity' is not defined
```

Component: **symbolics**

Author: **Ralf Stephan**

Branch/Commit: **[`bd4d2cf`](https://github.com/sagemath/sagetrac-mirror/commit/bd4d2cf752db056b09efba2c0f0c7a3a8d36fe0a)**

Reviewer: **Jeroen Demeyer**

_Issue created by migration from https://trac.sagemath.org/ticket/21906_

