Fixed out-of-bounds read in module protocol when a '%' line has no '='

[aizu-m]

AddressSanitizer, parsing a "%" module-protocol line with no "=":

ERROR: AddressSanitizer: SEGV on a READ
    #0 strlen
    #1 BufferNewFrom buffer.c:68
    #2 ModuleProtocol evalfunction.c:10252

Came across this whilst reading the module protocol parser. The "%" case
builds the JSON payload with

BufferNewFrom(line + strlen(name) + 1 + 1,
              length - strlen(name) - 1 - 1);

which assumes a well-formed "%name=" line. But sscanf("%256[^=]=",
name) also returns with name set when the line carries no "=": it leaves
the whole tail in name. So strlen(name) == length - 1, the length argument
underflows to SIZE_MAX, and the data pointer steps one past the terminating
NUL. BufferAppend() then scans from there and walks off the heap line
buffer.

read_module_protocol() hands arbitrary file lines straight to
ModuleProtocol(), so a single line "%x" is enough to reproduce it.

The other directives ("=", "@", ...) already guard their input; the "%"
case did not. Only do the arithmetic once the "=" delimiter is present.