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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