tool_formparse: avoid clobbering on function params

tool_formparse: avoid clobbering on function params

While perfectly legal to do, clobbering function parameters and using
them as local variables is confusing at best and rarely improves code
readability.  Fix by using a local variable instead, no functionality
is changed.

This also renames the parameter from data to mime_data since the term
data is (soft) reserved for the easy handle struct.

Closes: #10046
Reviewed-by: Daniel Stenberg <daniel@haxx.se>