Fix that `String#to_f` accepts consecutive `_` as a numeric expression

Consecutive `_` is not allowed as a numeric expression:

  1_2__3             #=> SyntaxError
  Float("1_2__3")    #=> ArgumentError
  Integer("1_2__3")  #=> ArgumentError
  "1_2__3".to_i      #=> 12

But `String#to_f` accept it, so I fixed the issue.

Before this patch:

  "1_2__3".to_f      #=> 123

After this patch:

  "1_2__3".to_f      #=> 12

1 files changed, 4 insertions, 9 deletions

diff --git a/src/string.c b/src/string.c
index 61fbd4ded..f75679513 100644
--- a/src/string.c
+++ b/src/string.c
@@ -2522,15 +2522,10 @@ bad:
     while (p < end && n < e) prev = *n++ = *p++;
     while (*p) {
       if (*p == '_') {
-        /* remove underscores between digits */
-        if (badcheck) {
-          if (n == buf || !ISDIGIT(prev)) goto bad;
-          ++p;
-          if (!ISDIGIT(*p)) goto bad;
-        }
-        else {
-          while (*++p == '_');
-          continue;
+        /* remove an underscore between digits */
+        if (n == buf || !ISDIGIT(prev) || (++p, !ISDIGIT(*p))) {
+          if (badcheck) goto bad;
+          break;
         }
       }
       prev = *p++;