Connector/C Out-of-bounds read in `unpack_fields()` from short metadata field

`unpack_fields()` in `libmariadb/mariadb_lib.c` parses 12 bytes of fixed-header metadata (charsetnr, length, type, flags, decimals) from each column descriptor without checking that the field data is present or long enough. A malicious server can send a truncated metadata field (0-11 bytes instead …

p= (char *)row->data[i];
/* filler */
field->charsetnr= uint2korr(p);
p+= 2;
field->length= (uint) uint4korr(p);
p+= 4;
field->type= (enum enum_field_types)uint1korr(p);
p++;
field->flags= uint2korr(p);
p+= 2;
field->decimals= (uint) p[0];
p++;
/* filler */
p+= 2;