loop: Error on invalid by constant

[kpoeck]

• solves Loop for by should only allow a positive number #300
• This creates an error while macroexpanding. I believe this is correct, but please check.
• Did run the ccl-test aber building a heap image with this change:

? (run-tests)
; Warning: Redefining test CCL.ISSUE\#167
; While executing: REGRESSION-TEST::REPORT-ERROR, in process listener(1).
Doing 21908 pending tests of 21908 tests total.
Invoking restart: #<RESTART CL-TEST::FOO #x133C12D>
Invoking restart: #<RESTART CL-TEST::FOO #x133C12D>
Invoking restart: #<RESTART CL-TEST::FOO #x133C12D>
Invoking restart: #<RESTART CL-TEST::FOO #x133C12D>
Invoking restart: #<RESTART CL-TEST::FOO #x133C12D>

=============== All tests succeeded ===============

(FUNCALL DO-TESTS :COMPILE COMPILE :VERBOSE VERBOSE :CATCH-ERRORS T)
took 160,935,936 microseconds (160.935940 seconds) to run.
       2,345,602 microseconds (  2.345602 seconds, 1.46%) of which was spent in GC.
During that period, and with 8 available CPU cores,
     151,279,803 microseconds (151.279800 seconds) were spent in user mode
       9,532,592 microseconds (  9.532592 seconds) were spent in system mode
 7,716,825,024 bytes of memory allocated.
 22,427 minor page faults, 10 major page faults, 0 swaps.
NIL