Avoid type-unsafe use of 'head'

Since the value had come out of split, and the inputs are sanitised
by argparse, this wasn't an acutal issue, but it was generating some
CI warnings.

src/RISCV/ArchDesc.hs

@@ -150,17 +150,19 @@ fromString str = ArchDesc { has_xlen_32     = True
                           }
   where rawSplit = splitOneOf "_zx" (map toLower str)
         archStrings = filter (\x -> not $ null x) rawSplit
-        rv64 = (head archStrings) =~ "rv64"
-        i = (head archStrings =~ "i") || (head archStrings =~ "g")
-        m = (head archStrings =~ "m") || (head archStrings =~ "g")
-        s = (head archStrings =~ "s") || (head archStrings =~ "g")
-        a = (head archStrings =~ "a") || (head archStrings =~ "g")
-        f = (head archStrings =~ "f") || (head archStrings =~ "g")
-        d = (head archStrings =~ "d") || (head archStrings =~ "g")
-        icsr = elem "icsr" archStrings || (head archStrings =~ "g")
-        ihpm = elem "ihpm" archStrings || (head archStrings =~ "g")
-        ifencei = elem "ifencei" archStrings || (head archStrings =~ "g")
-        c = head archStrings =~ "c"
-        n = head archStrings =~ "n"
+        archHead = case archStrings of []  -> ""
+                                       h:t -> h
+        rv64 = (archHead) =~ "rv64"
+        i = (archHead =~ "i") || (archHead =~ "g")
+        m = (archHead =~ "m") || (archHead =~ "g")
+        s = (archHead =~ "s") || (archHead =~ "g")
+        a = (archHead =~ "a") || (archHead =~ "g")
+        f = (archHead =~ "f") || (archHead =~ "g")
+        d = (archHead =~ "d") || (archHead =~ "g")
+        icsr = elem "icsr" archStrings || (archHead =~ "g")
+        ihpm = elem "ihpm" archStrings || (archHead =~ "g")
+        ifencei = elem "ifencei" archStrings || (archHead =~ "g")
+        c = archHead =~ "c"
+        n = archHead =~ "n"
         cheri = elem "cheri" archStrings
         nocloadtags = elem "nocloadtags" archStrings