BUG: Fix typos in conversion_utils

Unparenthesized logical expressions in if statements caused clang
compilation failure on OSX.
MAINT: multiarray: fix types

convert_datatype.c and number.c now compile without warnings.
MAINT: umath: get rid of "statement with no effect" warning

(What was this line for anyway?)
Merge pull request #4061 from larsmans/c-fixes

MAINT: Minor C fixes