rescue_parser: restructure code to avoid Coverity false positive

If line contains single word, line and argv[0] are aliases, so
no NULL dereference is possible, but Coverity does not know it.
Change code to avoid ambiguity and also remove redundant call to
grub_strchr.

CID: 86725

grub-core/kern/rescue_parser.c

@@ -43,14 +43,18 @@ grub_rescue_parse_line (char *line,
 
   /* In case of an assignment set the environment accordingly
      instead of calling a function.  */
-  if (n == 1 && grub_strchr (line, '='))
+  if (n == 1)
     {
       char *val = grub_strchr (args[0], '=');
+
+      if (val)
+	{
 	  val[0] = 0;
 	  grub_env_set (args[0], val + 1);
 	  val[0] = '=';
 	  goto quit;
 	}
+    }
 
   /* Get the command name.  */
   name = args[0];
@@ -72,6 +76,7 @@ grub_rescue_parse_line (char *line,
     }
 
  quit:
+  /* Arguments are returned in single memory chunk separated by zeroes */
   grub_free (args[0]);
   grub_free (args);