[ruby-cvs:64288] rhe:r57122 (trunk): string.c: add missing size_t cast

[rhe at ruby-lang.org]

rhe	2016-12-20 15:53:45 +0900 (Tue, 20 Dec 2016)

  New Revision: 57122

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=57122

  Log:
    string.c: add missing size_t cast
    
    Add size_t cast to avoid signed integer overflow. r56157 ("string.c:
    avoid signed integer overflow", 2016-09-13) missed this. Suppresses
    UBSan.

  Modified files:
    trunk/string.c