m_getfld: Shorten fdelimlen by one; it was too long.

m_getfld: Shorten fdelimlen by one;  it was too long.

The fdelimlen bytes starting at fdelim included a NUL at the end, e.g. 7
for "\nFrom \0".  Other code seems not to expect this and has
conditional bits to work around it, perhaps without understanding why.
All the tests still pass, including with valgrind.