rolling back trace pc guards

fuzzers fail with:

=================================================================
�[1m�[31m==18057==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000b8a144 at pc 0x0000007ae0ca bp 0x7fff2b91a4d0 sp 0x7fff2b91a4c8
�[1m�[0m�[1m�[34mWRITE of size 4 at 0x000000b8a144 thread T0�[1m�[0m
    #0 0x7ae0c9 in fuzzer::TracePC::HandleInit(unsigned int*, unsigned int*) /src/libfuzzer/FuzzerTracePC.cpp:49:8
    #1 0x7bcab9 in __sanitizer_cov_trace_pc_guard_init /src/libfuzzer/FuzzerTracePC.cpp:286:15
    #2 0x5156bf in sancov.module_ctor (/out/curl_fuzzer+0x5156bf)
    #3 0x88c1cc in __libc_csu_init (/out/curl_fuzzer+0x88c1cc)
    #4 0x7f4ab7aed7be in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x207be)
    #5 0x41fd78 in _start (/out/curl_fuzzer+0x41fd78)

Author: mikea

infra/base-images/base-libfuzzer/Dockerfile

@@ -19,7 +19,7 @@ MAINTAINER mike.aizatsky@gmail.com
 RUN apt-get install -y git subversion jq zip python3
 
 ENV SANITIZER_FLAGS="-fsanitize=address"
-ENV COV_FLAGS="-fsanitize-coverage=trace-pc-guard"
+ENV COV_FLAGS="-fsanitize-coverage=edge,indirect-calls,8bit-counters"
 
 ENV ASAN_OPTIONS="symbolize=1:detect_leaks=0"
 ENV FUZZER_LDFLAGS "-Wl,-whole-archive /usr/local/lib/libc++.a /usr/local/lib/libc++abi.a -Wl,-no-whole-archive"