fix: tform: potential crash due to race when MODMAX/MODMIN

A worker could free a dollar variable's data, while another worker
is trying to read it. Lock the dollar variable before working out
its RHS in case the RHS depends on the dollar variable that is
being assigned.

Author: jodavies

sources/dollar.c

@@ -394,14 +394,11 @@ NoChangeOne:;
 	}
 /*
 	Now the real evaluation.
-	In the case of threads and MODSUM this requires an immediate lock.
-	Otherwise the lock could be placed later.
+	We need to lock here before we work out the RHS, in case the RHS itself
+	depends on the dollar variable.
 */
 #ifdef WITHPTHREADS
-	if ( dtype == MODSUM ) {
-/*		LOCK(d->pthreadslockwrite); */
-		LOCK(d->pthreadslockread);
-	}
+	LOCK(d->pthreadslockread);
 #endif
 	CleanDollarFactors(d);
 /*
@@ -445,12 +442,7 @@ NoChangeOne:;
 		}
 		numterms = 0; t = ss; while ( *t ) { numterms++; t += *t; }
 	}
-#ifdef WITHPTHREADS
-	if ( dtype != MODSUM ) {
-/*		LOCK(d->pthreadslockwrite); */
-		LOCK(d->pthreadslockread);
-	}
-#endif
+
 	if ( numterms == 0 ) {
 /*
 		the new value evaluates to zero