Use the proper code path to remove regions

... in fellow_busy_obj_trimstore():

The code did not work correctly when the region was to be removed
(because it was reduced to size zero) _and_ it was not the last of the
regions.

Part of the fix for #39

src/fellow_cache.c

@@ -4346,11 +4346,14 @@ fellow_busy_obj_trimstore(struct fellow_busy *fbo)
 		// fbr offset where fcs starts
 		base = fbr->len - fcs->alloc.size;
 		nsz = base + fcs->len;
-		fellow_region_trim(fc->ffd, fdr, nsz);
 
-		nsz = fdr->size;
 		if (nsz == 0)
-			fbo->nregion--;
+			fellow_busy_region_free(fbo, fdr);
+		else {
+			fellow_region_trim(fc->ffd, fdr, nsz);
+			nsz = fdr->size;
+			AN(nsz);
+		}
 
 		assert(fcs->alloc.size >= (nsz - base));
 		surplus = fcs->alloc.size - (nsz - base);