sh: Detect an error for ${#var<GARBAGE>}.

In particular, this makes things like ${#foo[0]} and ${#foo[@]} errors
rather than silent equivalents of ${#foo}.

PR:		bin/151720
Submitted by:	Mark Johnston
Exp-run done by: pav (with some other sh(1) changes)

bin/sh/parser.c

@@ -1569,6 +1569,8 @@ varname:
 				}
 			}
 		} else if (subtype != VSERROR) {
+			if (subtype == VSLENGTH && c != '}')
+				subtype = VSERROR;
 			pungetc();
 		}
 		STPUTC('=', out);

tools/regression/bin/sh/errors/bad-parm-exp6.2

@@ -0,0 +1,2 @@
+# $FreeBSD$
+${#foo^}

tools/regression/bin/sh/errors/bad-parm-exp6.2.stderr

@@ -0,0 +1 @@
+./errors/bad-parm-exp6.2: ${foo...}: Bad substitution