Add fuzzing of multipart_parser

LibFuzzer is in-process, coverage-guided, evolutionary fuzzing engine.

LibFuzzer is linked with the library under test, and feeds fuzzed inputs to
the library via a specific fuzzing entrypoint (aka "target function"); the
fuzzer then tracks which areas of the code are reached, and generates
mutations on the corpus of input data in order to maximize the code coverage.

Lets use libFuzzer to fuzz multipart_parser for the start.

Ref: https://llvm.org/docs/LibFuzzer.html
Signed-off-by: Petr Štetiar <ynezz@true.cz>