input: let the handler of string binds return a byte whenever possible

The function get_code_from_plantation() should return ERR only when
the string bind is fully exhausted.  In the normal case, where some
bytes are still available, it should return the first of these bytes,
so that the {verbatim} function will work too.

This fixes https://savannah.gnu.org/bugs/?63702.

Bug existed since version 7.0, commit 958ec294,
since command cartouches were introduced.

src/winio.c

@@ -383,6 +383,7 @@ int get_code_from_plantation(void)
 		return PLANTED_COMMAND;
 	} else {
 		char *opening = strchr(plants_pointer, '{');
+		char firstbyte = *plants_pointer;
 		int length;
 
 		if (opening) {
@@ -391,12 +392,12 @@ int get_code_from_plantation(void)
 		} else
 			length = strlen(plants_pointer);
 
-		for (int index = length - 1; index >= 0; index--)
+		for (int index = length - 1; index > 0; index--)
 			put_back((unsigned char)plants_pointer[index]);
 
 		plants_pointer += length;
 
-		return ERR;
+		return (firstbyte) ? firstbyte : ERR;
 	}
 }
 #endif