patch 8.2.3475: expression register set by not executed put command

Problem:    Expression register set by not executed put command.
Solution:   Do not set the register if the command is skipped. (closes #8909)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 617de6f..2c55e67 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2382,9 +2382,12 @@
 	    // for '=' register: accept the rest of the line as an expression
 	    if (ea.arg[-1] == '=' && ea.arg[0] != NUL)
 	    {
-		set_expr_line(vim_strsave(ea.arg), &ea);
+		if (!ea.skip)
+		{
+		    set_expr_line(vim_strsave(ea.arg), &ea);
+		    did_set_expr_line = TRUE;
+		}
 		ea.arg += STRLEN(ea.arg);
-		did_set_expr_line = TRUE;
 	    }
 #endif
 	    ea.arg = skipwhite(ea.arg);