fuzzers: ignore size limits when compiled standalone

This way we can still call fuzzers on old samples, but oss-fuzz will not waste
its and our time finding overly large inputs.

src/fuzz/fuzz-bootspec.c

@@ -84,7 +84,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_(boot_config_free) BootConfig config = BOOT_CONFIG_NULL;
         int r;
 
-        if (size > 65536)
+        if (outside_size_range(size, 0, 65536))
                 return 0;
 
         /* Disable most logging if not running standalone */

src/fuzz/fuzz-env-file.c

@@ -12,7 +12,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_strv_free_ char **rl = NULL, **rlp =  NULL;
 
-        if (size > 65536)
+        if (outside_size_range(size, 0, 65536))
                 return 0;
 
         f = data_to_file(data, size);

src/fuzz/fuzz.h

@@ -4,6 +4,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "env-util.h"
 #include "fileio.h"
 
 /* The entry point into the fuzzer */
@@ -15,3 +16,14 @@ static inline FILE* data_to_file(const uint8_t *data, size_t size) {
         else
                 return fmemopen_unlocked((char*) data, size, "re");
 }
+
+/* Check if we are within the specified size range.
+ * The upper limit is ignored if FUZZ_USE_SIZE_LIMIT is unset.
+ */
+static inline bool outside_size_range(size_t size, size_t lower, size_t upper) {
+        if (size < lower)
+                return true;
+        if (size > upper)
+                return FUZZ_USE_SIZE_LIMIT;
+        return false;
+}