src/libpcp/src/context.c: fix lock reversal (covscan)
src/libpcp/src/context.c: fix lock reversal (covscan)

Found by Coverity, CID 287892.

We were locking context_lock, then locking ctxp->c_lock and later in
waitawhile() locking context_lock while still holding ctxp->c_lock.

Now we unlock context_lock before locking ctxp->c_lock ...  which is
safe because even if contexts[] is realloc'd by another thread,
all of the contexts[i] values (and hence ctxp) remain the same.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp