Fix error handling string memory leak

[jecassis]

Through the use of the CRT debugger while developing foo_vis_milk2 I uncovered a small heap leak in the error handling which is not releasing the previous parser error string before storing a new one.

In "CompilerFunctions.c", prjm_eval_error() duplicates each parse error message with _strdup() and assigns it to cctx->error.error.
When multiple parse errors occur in the same compile context, each new assignment overwrites the previous pointer without freeing it first.

Here is the CRT output (on Windows):

Detected memory leaks!
Dumping objects ->
projectm-eval\CompilerFunctions.c(22) : {1428482} normal block at 0x000001E72F2BD2C0, 29 bytes long.
 Data: <syntax error, un> 73 79 6E 74 61 78 20 65 72 72 6F 72 2C 20 75 6E 
projectm-eval\CompilerFunctions.c(22) : {1408261} normal block at 0x000001E757B0CE00, 29 bytes long.
 Data: <syntax error, un> 73 79 6E 74 61 78 20 65 72 72 6F 72 2C 20 75 6E 

I tested the following fix:

void prjm_eval_error(PRJM_EVAL_LTYPE* loc, prjm_eval_compiler_context_t* cctx, yyscan_t yyscanner, char const* s)
{
    free(cctx->error.error);
    cctx->error.error = _strdup(s);
...

Before assigning a new error message, it frees the previous cctx->error.error in prjm_eval_error(). Consequently, each new parser error replaces the previous message without losing ownership of allocated memory.

free(NULL) is valid, so this is safe for first-use and unchanged behavior for successful parses.

This should result in no functional behavior delta and, since making this change, I have not seen the memory leaks reoccurring.