jemalloc: Clamp in malloc_usable_size() only if a tag is present

This behaviour should be safe and is useful for testing whether the bounds
applied by jemalloc match what malloc_usable_size() reports.  That is, one
can validate bounds with a check like,

    assert(cheri_getlen(p) == malloc_usable_size(cheri_cleartag(p)));

In particular, malloc_usable_size() will look up the usable size using
allocator metadata, so it's handy to be able to compare that size with the
capability bounds.

contrib/jemalloc/src/jemalloc.c

@@ -3770,7 +3770,7 @@ je_malloc_usable_size(JEMALLOC_USABLE_SIZE_CONST void *ptr) {
 			ret = isalloc(tsdn, ptr);
 		}
 #ifdef __CHERI_PURE_CAPABILITY__
-		if (ret != 0) {
+		if (ret != 0 && cheri_gettag(ptr)) {
 			ret = MIN(ret, cheri_getlen(ptr));
 		}
 #endif