parser: Fail gracefully if a nonexistent file is passed on the command line

Before, if a nonexistent file was passed to LoadFileText(), it would
return NULL, and the parser would happily dereference it.

parser/raylib_parser.c

@@ -207,6 +207,12 @@ int main(int argc, char* argv[])
     int length = 0;
     char *buffer = LoadFileText(inFileName, &length);
 
+    if (buffer == NULL)
+    {
+	    printf("Could not read input file: %s\n", inFileName);
+	    return 1;
+    }
+
     // Preprocess buffer to get separate lines
     // NOTE: GetTextLines() also removes leading spaces/tabs
     int linesCount = 0;