pretty.c: make git_pretty_formats_config return -1 on git_config_string failure

pretty.c: make git_pretty_formats_config return -1 on git_config_string failure

`git_pretty_formats_config()` continues without checking git_config_string's
return value which can lead to a SEGFAULT. Instead return -1 when
git_config_string fails signalling `git_config()` to die printing the location
of the erroneous variable.

Signed-off-by: Tanay Abhra <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>