Fix GM 8.1 bug with get_function_address() segfault on scripts and extension functions

[cher-nov]

If you call get_function_address() with the name of a function from a dynamic library included by a GEX extension or a script (including ones from extensions), then at best, 0 or a garbage value may be returned, and at worst, an Access Violation error may occur.

The preferred fix for the get_function_address() is to simply return -1 in these cases, as it does for non-existent names.