seg fault with EXTRA_PREREQS

Seems that using .EXTRA_PREREQS can cause a segfault under the right
conditions.

 in file.c, when"snap_deps" is called to process the EXTRA_PREREQS..

    struct dep *prereqs = expand_extra_prereqs (lookup_variable
(STRING_SIZE_TUPLE(".EXTRA_PREREQS")));

    /* Perform per-file snap operations.  */
    hash_map_arg(&files, snap_file, prereqs);

The "expand_extra_prereqs" function is allowed to insert values in the hash
table, and this can invalidate the loop in hash_map_arg

stack trace of the call:

snap_deps
hash_map_arg   (loop happens here)
snap_file
expand_extra_prereqs
enter_file
hash_insert_at
hash_rehash  (loop no longer valid)

causes a fairly consistent crash.  I'm working on a minimal reproduction if
it's necessary, but allowing the table to be rehashed while iterating seems
like a problem.