string_number_format: Correctly handles return value of snprintf

Summary: `snprintf` can return a value greater than the number of bytes copied. In case the first byte of the string is not a digit (could be '-'), size of `tmpstr` was being updated without checking `tmplen`. This resulted in either an assertion error or a heap overflow depending on whether the assertion is compiled or not.

Reviewed By: mofarrell, qianxuweiren

Differential Revision: D17327899

fbshipit-source-id: ee53875d21e02608c6d870388eecf1464de24ff1