Potential heap overflow in `util_strip_whitespace`

This is caused by the len > 0 check on line 13 which comes after the array access but len can be 0 already which will cause an out-of-bounds access past the string.

https://github.com/p2sr/mdp/blob/4a580f63eb5c70bd1371153f2ff1ad219a1598fa/src/util.c#L8C1-L17

Found with libFuzzer + ASAN: CFLAGS=-Wall -Werror -fsanitize=address,fuzzer LDFLAGS=-lm -fsanitize=address,fuzzer

Reproducible example:

#include "util.h"

extern int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
    char* str = (char*)malloc(Size + 1);
    strncpy(str, (char*)Data, Size);
    str[Size] = 0;
    util_strip_whitespace(str);
    free(str);
    return 0;
}

SUMMARY: AddressSanitizer: heap-buffer-overflow (/home/nekz/git/clone/mdp/mdp+0x53764c) in util_strip_whitespace
Shadow bytes around the buggy address:
  0x0c047fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c047fff8000: fa fa 01 fa fa[fa]01 fa fa fa fa fa fa fa fa fa
  0x0c047fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==419636==ABORTING
MS: 0 ; base unit: 0000000000000000000000000000000000000000

p2sr / mdp

Potential heap overflow in `util_strip_whitespace` #7