seq: Fix handling of 0's and invalid numbers

Also added some corresponding testcases.

Author: Expertcoderz

tests/seq.sh

@@ -3,6 +3,9 @@
 set -eux
 
 set -- \
+  '0' \
+  '0 0' \
+  '0 0 0' \
   '10' \
   '5 10' \
   '0 2 10' \

userland/utilities/seq.py

@@ -59,9 +59,12 @@ def arg_to_decimal(arg: str) -> Decimal:
     exponent: int
 
     if len(args) == 1:
+        last = arg_to_decimal(args[0])
+        if not last:
+            return 0
+
         first = Decimal(1)
         increment = Decimal(1)
-        last = arg_to_decimal(args[0])
         exponent = 0
     elif len(args) == 2:
         first = arg_to_decimal(args[0])
@@ -81,6 +84,9 @@ def arg_to_decimal(arg: str) -> Decimal:
             else increment
         )
 
+    if not increment:
+        parser.error(f"invalid zero increment value: '{increment}'")
+
     formatstr: str
 
     if opts.equal_width: