Bug#585393: Control file parsing broken by comment and blank line

[Dee Holtsclaw (BLOOMBERG/ 919 3RD A)]

If a debian/control file containing a comment line followed by a blank line is processed by Sources.iter_paragraphs with use_apt_pkg=False, it silently fails to iterate stanzas. Processing the same file with Packages.iter_paragraphs succeeds. Stripping either the comment or the blank line succeeds.

Failing example:

```
# A comment 

Source: abcd
Section: unknown
Priority: extra
Build-Depends: xyzzy
```

Successful example:
```
# A comment 
Source: abcd
Section: unknown
Priority: extra
Build-Depends: xyzzy
```

We are using version 1.40-3
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