libproxy: buffer overflow when PAC is enabled (CVE-2020-26154)

url.cpp in libproxy through 0.4.15 is prone to a buffer overflow when PAC is enabled, as demonstrated by a large PAC file that is delivered without a Content-length header.

References:

• https://github.com/libproxy/libproxy/pull/126
• https://nvd.nist.gov/vuln/detail/CVE-2020-26154

Affected branches:

• master
• 3.12-stable