Fix VLA error in strings unittest (#3632)
Fix VLA error in strings unittest (#3632)

In the test-api-string.c there was an error if the unittests are built in release.
Full error message:
```
tests/unit-core/test-api-strings.c:89:20: error: variable-length array bound is unknown [-Werror=vla-larger-than=]
89 |   JERRY_VLA (char, string_from_cesu8_string, cesu8_sz);
```

The problem is that the compiler does not know if the buffer size is always greater than zero.
Added an extra check to the TEST_ASSERT a line above to resolve the error.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com
dbatyai/jerryscript,akosthekiss/jerryscript,jerryscript-project/jerryscript,dbatyai/jerryscript,bzsolt/jerryscript,jerryscript-project/jerryscript,robertsipka/jerryscript,akosthekiss/jerryscript,zherczeg/jerryscript,robertsipka/jerryscript,dbatyai/jerryscript,akosthekiss/jerryscript,dbatyai/jerryscript,robertsipka/jerryscript,jerryscript-project/jerryscript,akosthekiss/jerryscript,zherczeg/jerryscript,jerryscript-project/jerryscript,zherczeg/jerryscript,akosthekiss/jerryscript,bzsolt/jerryscript,bzsolt/jerryscript,robertsipka/jerryscript,jerryscript-project/jerryscript,zherczeg/jerryscript,dbatyai/jerryscript,robertsipka/jerryscript,zherczeg/jerryscript,bzsolt/jerryscript,robertsipka/jerryscript,bzsolt/jerryscript