Check for backtrace_fd returning NULL

src/backtrace.c

@@ -35,6 +35,7 @@ void uc_backtrace_buf(char *buf, size_t len)
     buf[0] = 0;
 
     lines = backtrace_symbols(buffer, ptrs);
+    if (lines != NULL) {
         for (i = 1; i < ptrs; i++) {
 
             int rc = snprintf(buf, remaining, "%s\n", lines[i]);
@@ -46,6 +47,7 @@ void uc_backtrace_buf(char *buf, size_t len)
             remaining -= rc;
             buf += rc;
         }
+    }
 
     free(lines);
 }