connect: make sure that rc is initialized in singleipconnect()

This commit fixes a Clang warning introduced in curl-7_48_0-190-g8f72b136:

Error: CLANG_WARNING:
lib/connect.c:1120:11: warning: The right operand of '==' is a garbage value
1118|       }
1119|
1120|->     if(-1 == rc)
1121|         error = SOCKERRNO;
1122|     }