linux-yocto: check for bad SRCREV before branch checks

linux-yocto: check for bad SRCREV before branch checks

do_validate_branches ensures that the desired SRCREV is at
the tip of every branch that contains the revision. To do this,
it looks for containing branches and processes them.

This processing was mistakenly placed before the check for an
invalid SRCREV, hence a git error message is seen in the log
if a bad SRCREV is used, rather than a clear message.

reordering the checks, and fixing a check for master, ensures
that clear messages are generated in all cases.

(From OE-Core rev: 7e4518bf88af53b09536a3bafcd4c392a094023f)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>