Fix mass re-orphaning of cap-gains splits during lot scrub

xaccSplitComputeCapGains() performs a marker->source switch-over when a
gains marker split is processed before its source. The switch-over did
not resolve the source's in-memory gain status, so the source was left
with gains==UNKNOWN and gains_split==nullptr even though its forward
gains-split KVP was intact. The subsequent lookup read nullptr and the
code created a brand-new gains transaction routed to Orphaned Gains,
duplicating hundreds of already-settled gains splits on SQL books whose
history contains a stock split.

Resolve the source's gain status immediately after the switch-over so
its existing gains split is reused instead of being re-created.

This latent bug (original cap-gains code, ~2003) was exposed by the
5.17 adjusted_amount work (1e654412ab, b1efe0763c) which recomputes
already-booked gains in stock-split accounts. It is order-sensitive,
which is why it reproduces on MySQL but not SQLite.

libgnucash/engine/cap-gains.cpp

@@ -609,6 +609,12 @@ xaccSplitComputeCapGains(Split *split, Account *gain_acc)
 #endif
         }
         split = s;
+        /* The source reached by switch-over may not have resolved its own
+         * gain status yet (e.g. the marker was scrubbed before its source);
+         * resolve it now so its existing gains split is reused instead of
+         * being re-created and orphaned. */
+        if (GAINS_STATUS_UNKNOWN == split->gains)
+            xaccSplitDetermineGainStatus (split);
     }
 
     /* Note: if the value of the 'opening' split(s) has changed,