Improve fuzz-conv fuzzer

Input format
 - Less restriction on input length
 - Input text and charset no longer overlapped
Performance
 - Less memory allocation
 - Less disk IO
 - Do not force GC. This make the fuzzer roughly 10x faster

fuzz/fuzz-conv.c

@@ -1,21 +1,9 @@
 #include <stdint.h>
-#include <string.h>
 #include <stdlib.h>
-#include <unistd.h>
 #include <gc.h>
 #include "wc.h"
 #include "wtf.h"
 
-char *get_null_terminated(const uint8_t *data, size_t size) {
-    char *new_str = (char *)malloc(size+1);
-    if (new_str == NULL){
-	exit(1);
-    }
-    memcpy(new_str, data, size);
-    new_str[size] = '\0';
-    return new_str;
-}
-
 static void *die_oom(size_t bytes) {
     fprintf(stderr, "Out of memory: %lu bytes unavailable!\n", (unsigned long)bytes);
     exit(1);
@@ -42,46 +30,27 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size){
 	init_done = 1;
     }
 
-    if (size < 30) {
-        return 0;
-    }
-
-    GC_disable();
-
-    char *new_str1 = get_null_terminated(data, 20);
-    data += 20; size -= 20;
-    char *new_str2 = get_null_terminated(data, size);
+    /* Assume the data format is:
+     *   <str1> \0 <str2> \0 <str3>
+     */
+    const uint8_t *str1, *str2, *str3;
+    const uint8_t *p;
+    str1 = data;
+    p = memchr(str1, '\0', size);
+    if (p == NULL) return 0;
+    str2 = p + 1;
+    if (str2 >= data + size) return 0;
+    p = memchr(str2, '\0', data + size - str2);
+    if (p == NULL) return 0;
+    str3 = p + 1;
 
     wc_ces old, from, to;
-    from = wc_guess_charset_short(new_str1,0);
-    to = wc_guess_charset_short(new_str2, 0);
+    from = wc_guess_charset_short((char*)str1, 0);
+    to = wc_guess_charset_short((char*)str2, 0);
 
-    char filename[256];
-    sprintf(filename, "/tmp/libfuzzer.%d", getpid());
-
-    FILE *fp = fopen(filename, "wb");
-    if (fp) {
-	fwrite(data, size, 1, fp);
-	fclose(fp);
-    }
-
-    FILE *f = fopen(filename, "r");
-    if (f) {
-	Str s = Strfgetall(f);
-	wc_Str_conv_with_detect(s, &from, from, to);
-	if (s != NULL) {
-	    Strfree(s);
-	}
-	fclose(f);
-    }
-
-    unlink(filename);
-
-    free(new_str1);
-    free(new_str2);
-
-    GC_enable();
-    GC_gcollect();
+    Str s = Strnew_charp_n((char*)str3, data + size - str3);
+    wc_Str_conv_with_detect(s, &from, from, to);
+    Strfree(s);
 
     return 0;
 }