Convert evbuffer_strspn() (internal helper) to use size_t

Convert evbuffer_strspn() (internal helper) to use size_t

As pointed by @yankeehacker in #590:
  Signed to Unsigned Conversion Error - buffer.c:1623

  Description: This assignment creates a type mismatch by populating an
  unsigned variable with a signed value. The signed integer will be
  implicitly cast to an unsigned integer, converting negative values into
  positive ones. If an attacker can control the signed value, it may be
  possible to trigger a buffer overflow if the value specifies the length
  of a memory write.

  Remediation: Do not rely on implicit casts between signed and unsigned
  values because the result can take on an unexpected value and violate
  weak assumptions made elsewhere in the program.

Fixes: #590