fix regression in user-specified source RPM

A recent commit 74316588e is unconditionally setting the SRCRPM path
overwriting a user specified path.

Only set SRCRPM if SRCRPM is not already set.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Merge pull request #429 from spartacus06/fix-srpm-regression

fix regression in user-specified source RPM