sh-script: Mark + and * as punctuation rather than a symbol constituent

In Bash, the characters + and * are not valid in variable names.
In sh, they are also disallowed in both function and variable
names.  Treating them as punctuation is appropriate, as they most
commonly appear as operators.

Examples:
   . Arithmetic: $((var1*var2))
   . Compound assignment: list_name+=("item")

* lisp/progmodes/sh-script.el (sh-mode-syntax-table): Mark + and *
as punctuation in the syntax table.  (Bug#80280)

Copyright-paperwork-exempt: yes

lisp/progmodes/sh-script.el

@@ -406,6 +406,8 @@ name symbol."
 	;; to work fine. This is needed so that dabbrev-expand
 	;; $VARNAME works.
 	?$ "'"
+	?* "."
+	?+ "."
 	?! "."
 	?% "."
 	?: "."