conf: use of NULL-pointer in DetectLoadCompleteSigPath

The "sig_file" argument of DetectLoadCompleteSigPath() is not checked for NULL-values. If this argument is NULL a SEGV occurs because of a dereferenced NULL-pointer in strlen in PathIsAbsolute. This commit fixes bug #2347. Here is the ASAN-output:

==17170==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fd1afa00646 bp 0x7ffe8398e6d0 sp 0x7ffe8398de58 T0)
    0 0x7fd1afa00645 in strlen (/lib/x86_64-linux-gnu/libc.so.6+0x80645)
    1 0x7fd1b3242eec  (/usr/lib/x86_64-linux-gnu/libasan.so.3+0x3beec)
    2 0x5561c8cddf7f in PathIsAbsolute /root/suricata-1/src/util-path.c:40
    3 0x5561c8cddfea in PathIsRelative /root/suricata-1/src/util-path.c:65
    4 0x5561c89275e4 in DetectLoadCompleteSigPath /root/suricata-1/src/detect.c:264
    5 0x5561c8929e75 in SigLoadSignatures /root/suricata-1/src/detect.c:486
    6 0x5561c8c0f2b3 in LoadSignatures /root/suricata-1/src/suricata.c:2419
    7 0x5561c8c1051d in PostConfLoadedDetectSetup /root/suricata-1/src/suricata.c:2550
    8 0x5561c8c12424 in main /root/suricata-1/src/suricata.c:2887
    9 0x7fd1af9a02b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    10 0x5561c87b31a9 in _start (/usr/local/bin/suricata+0xc51a9)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib/x86_64-linux-gnu/libc.so.6+0x80645) in strlen

src/detect-engine-loader.c

@@ -63,6 +63,11 @@ char *DetectLoadCompleteSigPath(const DetectEngineCtx *de_ctx, const char *sig_f
     char *path = NULL;
     char varname[128];
 
+    if (sig_file == NULL) {
+        SCLogError(SC_ERR_INVALID_ARGUMENTS,"invalid sig_file argument - NULL");
+        return NULL;
+    }
+
     if (strlen(de_ctx->config_prefix) > 0) {
         snprintf(varname, sizeof(varname), "%s.default-rule-path",
                 de_ctx->config_prefix);