flag: Check for Set error

Check for Set error when a boolean flag isn't explicitly given a value.

Fixes #9345

Change-Id: I97a1289f8cf27567d1a726ebe5ef167c800f357c
Reviewed-on: https://go-review.googlesource.com/1897Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -767,7 +767,9 @@ func (f *FlagSet) parseOne() (bool, error) {
 				return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
 			}
 		} else {
-			fv.Set("true")
+			if err := fv.Set("true"); err != nil {
+				return false, f.failf("invalid boolean flag %s: %v", name, err)
+			}
 		}
 	} else {
 		// It must have a value, which might be the next argument.