Set "max" to a reasonable value if BLOCKSIZE has a bogus unit. Prior

to this commit, "env BLOCKSIZE=4X df" prints not only "4X: unknown
blocksize" as expected, but sometimes also "maximum blocksize is 1G"
and "minimum blocksize is 512" depending on what happened to be on
the stack.

Found by:	LLVM/Clang Static Checker

lib/libc/gen/getbsize.c

@@ -83,6 +83,7 @@ getbsize(headerlenp, blocksizep)
 		default:
 fmterr:			warnx("%s: unknown blocksize", p);
 			n = 512;
+			max = MAXB;
 			mul = 1;
 			break;
 		}