gas: adjust a comparison in s_align()

In 344b1e0f5f79 ("gas: range-check 3rd argument of .align et al") I
neglected to consider compilers which warn about signed/unsigned
mismatches in comparisons (which is somewhat odd when the signed value is
already known to be non-negative).

gas/read.c

diff --git a/gas/read.c b/gas/read.c
index 0b817b7..a73e814 100644
--- a/gas/read.c
+++ b/gas/read.c
@@ -1600,7 +1600,7 @@
 	  ++input_line_pointer;
 	  offsetT val = get_absolute_expression ();
 	  max = val;
-	  if (val < 0 || max != val)
+	  if (val < 0 || max != (valueT) val)
 	    {
 	      as_warn (_("ignoring out of range alignment maximum"));
 	      max = 0;