VM: make FACTOR_FOR_EACH not call .end() more than once

vm/utilities.hpp

@@ -1,9 +1,10 @@
 namespace factor {
 
 // Poor mans range-based for loops.
-#define FACTOR_FOR_EACH(iterable)                           \
-  for (typeof((iterable).begin()) iter = (iterable).begin();    \
-       iter != (iterable).end();                                \
+#define FACTOR_FOR_EACH(iterable)                               \
+  for (typeof((iterable).begin()) iter = (iterable).begin(),    \
+           _end = (iterable).end();                             \
+       iter != _end;                                            \
        iter++)
 
 inline static void memset_2(void* dst, uint16_t pattern, size_t size) {