Handle input errors for --indent

Malformed and overflowing arguments were not caught. Also, skip spaces
after the number to match the spaces skipped before the number.

src/main.c

@@ -432,12 +432,16 @@ int main(int argc, char* argv[]) {
             fprintf(stderr, "%s: --indent takes one parameter\n", progname);
             die();
           }
-          dumpopts &= ~(JV_PRINT_TAB | JV_PRINT_INDENT_FLAGS(7));
-          int indent = atoi(argv[i+1]);
-          if (indent < -1 || indent > 7) {
+          char* end = NULL;
+          errno = 0;
+          long indent = strtol(argv[i+1], &end, 10);
+          while (*end && isspace((unsigned char)*end))
+            end++;
+          if (errno || indent < -1 || indent > 7 || *end) {
             fprintf(stderr, "%s: --indent takes a number between -1 and 7\n", progname);
             die();
           }
+          dumpopts &= ~(JV_PRINT_TAB | JV_PRINT_INDENT_FLAGS(7));
           dumpopts |= JV_PRINT_INDENT_FLAGS(indent);
           i++;
         } else if (isoption(&text, 0, "seq", is_short)) {