Fix findParameterStart() to correctly skip method return types.

findParameterStart() is used to move through a method's typeEncoding
string to the start of an indexed parameter's type specifier.
Because the typeEncoding begins with the return type,
findParameterStart() must move past the first type specifier
before it begins counting its way through the remaining ones.

Without this fix, method_getArgumentType() and method_copyArgumentType()
both return types that are off-by-one. These two functions are the
only callers of findParameterStart().