strftime/1: fix validation of non-string argument with number input

There was a incorrect else, that caused jq to not ensure that the
argument to strftime/1 is a string when the input is a number; this ends
up calling jv_string_value on a non-string value, which does not work,
and causes an assert failure.

Also fix same bug in strflocaltime/1.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67403

tests/jq.test

@@ -1585,6 +1585,15 @@ try mktime catch .
 ["a",1,2,3,4,5,6,7]
 "mktime requires parsed datetime inputs"
 
+# oss-fuzz #67403: non-string argument with number input fails assert
+try ["OK", strftime([])] catch ["KO", .]
+0
+["KO","strftime/1 requires a string format"]
+
+try ["OK", strflocaltime({})] catch ["KO", .]
+0
+["KO","strflocaltime/1 requires a string format"]
+
 # module system
 import "a" as foo; import "b" as bar; def fooa: foo::a; [fooa, bar::a, bar::b, foo::a]
 null